Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

A short-haul aircraft in the United Kingdom recently became the first airborne platform to test delicate quantum technologies that could usher in a post-GPS world—in which satellite-based navigation (be it GPS, BeiDou, Galileo, or others) cedes its singular place as a trusted navigational tool. The question now is how soon will it take for this quantum tomorrow to actually arrive.

But is this tech just around the corner, as its proponents suggest? Or will the world need to wait until the 2030s or beyond, as skeptics maintain. Whenever the technology can scale up, potential civilian applications will be substantial.

“The very first application or very valuable application is going to be autonomous shipping,” says Max Perez, vice president for strategic initiatives at the Boulder, Colo.–based company Infleqtion. “As we get these systems down smaller, they’re going to start to be able to address other areas like autonomous mining, for example, and other industrial settings where GPS might be degraded. And then, ultimately, the largest application will be generalized, personal autonomous vehicles—whether terrestrial or air-based.”

The big idea Infleqtion and its U.K. partners are testing is whether the extreme sensitivity that quantum sensors can provide is worth the trade-off of all the expensive kit needed to miniaturize such tech so it can fit on a plane, boat, spacecraft, car, truck, or train.

Turning Bose-Einstein Condensates Into Navigational Tools

At the core of Infleqtion’s technology is a state of matter called a Bose-Einstein condensate (BEC), which can be made to be extremely sensitive to acceleration. And in the absence of an external GPS signal, an aircraft that can keep a close tally on its every rotation and acceleration is an aircraft that can infer its exact location relative to its last known position.

As Perez describes it—the company has not yet published a paper on its latest, landmark accomplishment—Infleqtion’s somewhat-portable BEC device occupies 8 to 10 rack units of space. (One rack unit represents a standard server rack’s width of 48.3 centimeters and a standard server rack depth of 60–100 cm.)

Scientists tested delicate Bose-Einstein condensates in their instruments, which could one day undergird ultrasensitive accelerometers.Qinetiq

In May, the company flew its rig aboard a British Aerospace 146 (BAe 146/Avro RJ100) tech demonstrator aircraft. Inside the rig, a set of lasers blasted a small, supercooled cloud of rubidium atoms to establish a single quantum state among the atoms. The upshot of this cold atom trap is to create ultrasensitive quantum conditions among the whole aggregation of atoms, which is then a big enough cloud of matter to be able to be manipulated with standard laboratory equipment.

Using the quantum wave-particle duality, in which matter behaves both like tiny billiard balls and wave packets, engineers can then use lasers and magnetic fields to split the BEC cloud into two or more coherent matter-wave packets. When later recombined, the interference patterns of the multiple wave packets are studied to discover even the most minuscule accelerations—tinier than conventional accelerometers could measure—to the wave packets’ positions in three-dimensional space.

That’s the theoretical idea, at least.

Real-World Conditions Muddy Timetables

In practice, any BEC-based accelerometer would need to at least match the sensitivity of existing, conventional accelerometer technologies.

“The best inertial systems in the world, based on ring laser gyroscopes, or fiber-optic gyroscopes, can...maintain a nautical mile of precision over about two weeks of mission,” Perez says. “That’s the standard.”

The Infleqtion rig has provided only a proof of principle for creating a manipulable BEC state in a rubidium cloud, Perez adds, so there’s no one-to-one comparison yet available for the quantum accelerometer technology. That said, he expects Infleqtion to be able to either maintain the same nautical-mile precision over a month or more mission time—or, conversely, increase the sensitivity over a week’s mission to something like one-tenth of a nautical mile.

The eventual application space for the technology is vast, says Doug Finke, chief content officer at the New York City–based market research firm Global Quantum Intelligence.

“Quantum navigation devices could become the killer application for quantum-sensing technology,” Finke says. “However, many challenges remain to reduce the cost, size, and reliability. But potentially, if this technology follows it similar path to what happened in computing, from room-size mainframes to something that fits inside one’s pocket, it could become ubiquitous and possibly even replace GPS later this century.”

The timeframe for such a takeover remains an unanswered question. “It won’t happen immediately due to the engineering challenges still to be resolved,” Finke says. “And the technology may require many more years to reach maturation.”

Dana Goward, president of the Alexandria, Va.–based Resilient Navigation and Timing Foundation, even ventures a prediction. “It will be 10 to 15 years at least before we see something that is practical for broad application,” he says.

Perez says that by 2026, Infleqtion will be testing the reliability of its actual accelerometer technology—not just setting up a BEC in midflight, as it did in May. “It’s basically trading off getting the technology out there a little faster versus something that is more precise for more demanding applications that’ll be just behind that,” Perez says.

UPDATE 4 June 2024: The story was updated to modify the accuracy estimate for the best inertial navigation systems today—from one nautical mile per one-week mission (as a previous version of this story stated) to one nautical mile per two-week mission.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Neutrons are subatomic particles that have no electric charge, unlike protons and electrons. That means that while the electromagnetic force is responsible for most of the interactions between radiation and materials, neutrons are essentially immune to that force.

Instead, neutrons are held together inside an atom’s nucleus solely by something called the strong force, one of the four fundamental forces of nature. As its name implies, the force is indeed very strong, but only at very close range — it drops off so rapidly as to be negligible beyond 1/10,000 the size of an atom. But now, researchers at MIT have found that neutrons can actually be made to cling to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there just by the strong force.

The new finding may lead to useful new tools for probing the basic properties of materials at the quantum level, including those arising from the strong force, as well as exploring new kinds of quantum information processing devices. The work is reported this week in the journal ACS Nano, in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro of the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe material properties using a method called neutron scattering, in which a beam of neutrons is focused on a sample, and the neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.

But until this new work, nobody thought that these neutrons might actually stick to the materials they were probing. “The fact that [the neutrons] can be trapped by the materials, nobody seems to know about that,” says Li, who is also a professor of materials science and engineering. “We were surprised that this exists, and that nobody had talked about it before, among the experts we had checked with,” he says.

The reason this new finding is so surprising, Li explains, is because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Pretty much everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, and we know that is very short-range. It is effective at a range of 10 to the minus 15 power,” or one quadrillionth, of a meter.

“It’s very small, but it’s very intense,” he says of this force that holds the nuclei of atoms together. “But what’s interesting is we’ve got these many thousands of nuclei in this neutronic quantum dot, and that’s able to stabilize these bound states, which have much more diffuse wavefunctions at tens of nanometers [billionths of a meter].  These neutronic bound states in a quantum dot are actually quite akin to Thomson’s plum pudding model of an atom, after his discovery of the electron.”

It was so unexpected, Li calls it “a pretty crazy solution to a quantum mechanical problem.” The team calls the newly discovered state an artificial “neutronic molecule.”

These neutronic molecules are made from quantum dots, which are tiny crystalline particles, collections of atoms so small that their properties are governed more by the exact size and shape of the particles than by their composition. The discovery and controlled production of quantum dots were the subject of the 2023 Nobel Prize in Chemistry, awarded to MIT Professor Moungi Bawendi and two others.

“In conventional quantum dots, an electron is trapped by the electromagnetic potential created by a macroscopic number of atoms, thus its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” says Cappellaro. “Similarly, in these nucleonic quantum dots, a single neutron can be trapped by a nanocrystal, with a size well beyond the range of the nuclear force, and display similar quantized energies.” While these energy jumps give quantum dots their colors, the neutronic quantum dots could be used for storing quantum information.

This work is based on theoretical calculations and computational simulations. “We did it analytically in two different ways, and eventually also verified it numerically,” Li says. Although the effect had never been described before, he says, in principle there’s no reason it couldn’t have been found much sooner: “Conceptually, people should have already thought about it,” he says, but as far as the team has been able to determine, nobody did.

Part of the difficulty in doing the computations is the very different scales involved: The binding energy of a neutron to the quantum dots they were attaching to is about one-trillionth that of previously known conditions where the neutron is bound to a small group of nucleons. For this work, the team used an analytical tool called Green’s function to demonstrate that the strong force was sufficient to capture neutrons with a quantum dot with a minimum radius of 13 nanometers.

Then, the researchers did detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal, as well as the nuclear spin polarizations of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and wires of the material as opposed to particles.

But Li says that actually creating such neutronic molecules in the lab, which among other things requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something that other researchers with the appropriate expertise will have to undertake.

Li notes that “artificial atoms” made up of assemblages of atoms that share properties and can behave in many ways like a single atom have been used to probe many properties of real atoms. Similarly, he says, these artificial molecules provide “an interesting model system” that might be used to study “interesting quantum mechanical problems that one can think about,” such as whether these neutronic molecules will have a shell structure that mimics the electron shell structure of atoms.

“One possible application,” he says, “is maybe we can precisely control the neutron state. By changing the way the quantum dot oscillates, maybe we can shoot the neutron off in a particular direction.” Neutrons are powerful tools for such things as triggering both fission and fusion reactions, but so far it has been difficult to control individual neutrons. These new bound states could provide much greater degrees of control over individual neutrons, which could play a role in the development of new quantum information systems, he says.

“One idea is to use it to manipulate the neutron, and then the neutron will be able to affect other nuclear spins,” Li says. In that sense, he says, the neutronic molecule could serve as a mediator between the nuclear spins of separate nuclei — and this nuclear spin is a property that is already being used as a basic storage unit, or qubit, in developing quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s one potential application.” He adds that this is “quite different from electromagnetics-based quantum information processing, which is so far the dominant paradigm. So, regardless of whether it’s superconducting qubits or it’s trapped ions or nitrogen vacancy centers, most of these are based on electromagnetic interactions.” In this new system, instead, “we have neutrons and nuclear spin. We’re just starting to explore what we can do with it now.”

Another possible application, he says, is for a kind of imaging, using neutral activation analysis. “Neutron imaging complements X-ray imaging because neutrons are much more strongly interacting with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition but even about the different isotopes of those elements. “A lot of the chemical imaging and spectroscopy doesn’t tell us about the isotopes,” whereas the neutron-based method could do so, he says.

The research was supported by the U.S. Office of Naval Research.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.

As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.

Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity.

Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.

The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.

“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper on this technique, which is published today in PRX Quantum.

His co-authors include his advisor and corresponding author, Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and professor of physics; as well as Alexandre Cooper, a senior research scientist at the University of Waterloo’s Institute for Quantum Computing; and Won Kyu Calvin Sun, a former researcher in Cappellaro’s group who is now a postdoc at the University of Illinois at Urbana-Champaign.

Diamond defects

To create NV centers, scientists implant nitrogen into a sample of diamond.

But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.

Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.

Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.

In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits.

They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin.

Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.

Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.

Controlling a chain of dark spins

This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.

“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.

To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses.

The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.

“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.

While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers.

In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.

This research is supported, in part, by the U.S. National Science Foundation and the Canada First Research Excellence Fund.

Electronic devices typically use the charge of electrons, but spin — their other degree of freedom — is starting to be exploited. Spin defects make crystalline materials highly useful for quantum-based devices such as ultrasensitive quantum sensors, quantum memory devices, or systems for simulating the physics of quantum effects. Varying the spin density in semiconductors can lead to new properties in a material — something researchers have long wanted to explore — but this density is usually fleeting and elusive, thus hard to measure and control locally.

Now, a team of researchers at MIT and elsewhere has found a way to tune the spin density in diamond, changing it by a factor of two, by applying an external laser or microwave beam. The finding, reported this week in the journal PNAS, could open up many new possibilities for advanced quantum devices, the authors say. The paper is a collaboration between current and former students of professors Paola Cappellaro and Ju Li at MIT, and collaborators at Politecnico of Milano. The first author of the paper, Guoqing Wang PhD ’23, worked on his PhD thesis in Cappellaro’s lab and is now a postdoc at MIT.

A specific type of spin defect known as a nitrogen vacancy (NV) center in diamond is one of the most widely studied systems for its potential use in a wide variety of quantum applications. The spin of NV centers is sensitive to any physical, electrical, or optical disturbance, making them potentially highly sensitive detectors. “Solid-state spin defects are one of the most promising quantum platforms,” Wang says, partly because they can work under ambient, room-temperature conditions. Many other quantum systems require ultracold or other specialized environments.

“The nanoscale sensing capabilities of NV centers makes them promising for probing the dynamics in their spin environment, manifesting rich quantum many body physics yet to be understood”, Wang adds. “A major spin defect in the environment, called P1 center, can usually be 10 to 100 times more populous than the NV center and thus can have stronger interactions, making them ideal for studying many-body physics.”

But to tune their interactions, scientists need to be able to change the spin density, something that had previously seldom been achieved. With this new approach, Wang says, “We can tune the spin density so it provides a potential knob to actually tune such a system. That’s the key novelty of our work.”

Such a tunable system could provide more flexible ways of studying the quantum hydrodynamics, Wang says. More immediately, the new process can be applied to some existing nanoscale quantum-sensing devices as a way to improve their sensitivity.

Li, who holds a joint appointment in MIT’s departments of Nuclear Science and Engineering and Materials Science and Engineering, explains that today’s computers and information processing systems are all based on the control and detection of electrical charges, but some innovative devices are beginning to make use of the property called spin. The semiconductor company Intel, for example, has been experimenting with new kinds of transistors that couple spin and charge, potentially opening a path to devices based on spintronics.

“Traditional CMOS transistors use a lot of energy,” Li says, “but if you use spin, as in this Intel design, then you can reduce the energy consumption by a lot.” The company has also developed solid-state spin qubit devices for quantum computing, and “spin is something people want to control in solids because it’s more energy efficient, and it’s also a carrier of quantum information.”

In the study by Li and his colleagues, the newly achieved level of control over spin density allows each NV center to act like a kind of atomic-scale “radar” that can both sense and control the nearby spins. “We basically use a particular NV defect to sense the surrounding electronic and nuclear spins. This quantum sensor reveals the nearby spin environment and how that’s affected dynamically by the charge flow, which in this case is pumped up by the laser,” Li says.

This system makes it possible to dynamically change the spin concentration by a factor of two, he says. This could ultimately lead to devices where a single point defect or a single atom could be the basic computational unit. “In the long run, a single point defect, and the localized spin and the localized charge on that single point defect, can be a computing logic. It can be a qubit, it can be a memory, it can be a sensor,” he says.

He adds that much work remains to develop this newly found phenomenon. “We’re not exactly there yet,” he says, but what they have demonstrated so far shows that they have “really pushed down the measurement and control of the spin and charge state of point defects to an unprecedented level. So, in the long run, I think this would support using individual defect, or a small number of defects, to become the information processing and sensing devices.”

In this work so far, Wang says, “we find this phenomenon and we demonstrate it,” but further work is needed to fully understand the physical mechanism of what is taking place in these systems. “Our next step is to dig more deeply into the physics, so we would like to know better what’s the underlying physical mechanism” behind the effects they see. In the long term, “with better understanding of these systems, we hope to explore more quantum simulation and sensing ideas, such as simulating interesting quantum hydrodynamics, and even transporting quantum information between different spin defects.”

The findings were made possible, in part, by the team’s development of a new wide-field imaging setup that allows them to measure many different spatial locations within the crystalline material simultaneously, using a fast single-photon detector array, combined with a microscope. “We are able to spatially image the density distribution over different spin species like a fingerprint, and the charge transport dynamics,” although that work is still preliminary, Wang says.

Although their work was done using lab-grown diamond, the principles could be applied to other crystalline solid-state defects, he says. NV centers in diamond have been attractive for research because they can be used at room temperature and they have already been well-studied. But silicon vacancy centers, donors in silicon, rare-earth ions in solids, and other crystal materials may have different properties that could turn out to be useful for particular kinds of applications.

“As information science progresses, eventually people will be able to control the positions and the charge of individual atoms and defects. That’s the long-term vision,” Li says. “If you can have every atom storing different information, it’s a much larger information storage and processing capability” compared to existing systems where even a single bit is stored by a magnetic domain of many atoms. “You can say it’s the ultimate limit of Moore’s Law: eventually going down to one defect or one atom.”

While some applications may require much more research to develop to a practical level, for some kinds of quantum sensing systems, the new insights can be quickly translated into real-world uses, Wang says. “We can immediately improve the quantum sensors’ performance based on our results,” he says.

“Overall, this result is very exciting for the field of solid-state spin defects,” says Chong Zu, an assistant professor of physics at Washington University in St. Louis, who specializes in quantum information but was not involved in this work. “In particular, it introduces a powerful approach of using charge ionization dynamics to continuously tune the local spin defect density, which is important in the context of applications of NV centers for quantum simulation and sensing.”

The research team included Changhao Li, Hao Tang, Boning Li, Francesca Madonini, Faisal Alsallom, and Won Kyu Calvin Sun, all at MIT; Pai Peng at Princeton University; and Federica Villa at the Politecnico de Milano, in Italy. The work was partly supported by the U.S. Defense Advanced Research Projects Agency.