Breakthrough discoveries of neutron molecules and quantum dots at MIT

Research shows neutrons can combine Nanoscale Clusters of atoms are called quantum dots. The discovery could provide insights into material properties and quantum effects.

Unlike protons and electrons, neutrons are uncharged subatomic particles. This means that while the electromagnetic force is responsible for most interactions between radiation and materials, neutrons are largely unaffected by this force.

Neutrons interact through the strong force

Instead, neutrons come together inside a atomAtomic nuclei are driven entirely by something called the strong force, one of the four fundamental forces of nature. As the name suggests, this force is indeed very powerful, but only at very close ranges – it decays so quickly that it becomes negligible at atomic sizes larger than 1/10,000th.But now, researchers MIT The researchers discovered that it is actually possible to make neutrons attach to particles called quantum dots, which are composed of tens of thousands of atomic nuclei, holding them there only by the strong force.

The new discovery could lead to useful new tools for probing fundamental properties of materials at the quantum level, including those produced by the strong force, and for exploring new types of quantum information-processing devices.The work was recently reported in the journal ACS Nanoin a paper by MIT graduate students Tang Hao and Wang Guoqing and MIT Nuclear Science and Engineering professors Li Ju and Paola Capellaro.

Applications in Materials Science

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

But before this new work, no one thought that these neutrons might actually stick to the materials they were detecting. “the fact is [the neutrons] “It’s possible to become trapped in materials, but no one seems to know about it,” said Li, a professor of materials science and engineering. “We were surprised that this condition existed, and among the experts we consulted, no one had talked about it before. It,” he said.

New quantum mechanical insights

What makes the new discovery so surprising, Lee explained, is that neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he said. “Almost everything is an electromagnetic interaction, but in this case, since neutrons have no charge, the interaction here is through the strong interaction, which we know is very short-range. Its effective range is from 10 to negative The 15th power is one trillionth of a meter.

“It’s very small, but very strong,” he said of the force that holds atomic nuclei together. “But what’s interesting is that we have thousands of nuclei in this neutron quantum dot, which stabilizes these bound states that have much more diffuse wave functions at tens of nanometers. [billionths of a meter]. These neutron-bound states in quantum dots are actually very similar to the plum pudding model of atoms that followed Thomson’s discovery of the electron.

It was so unexpected that Li called it “a really crazy solution to a quantum mechanical problem.” The research team calls the newly discovered state artificial “neutron molecules.”

These neutron molecules are made from quantum dots, which are tiny crystalline particles, collections of very small atoms whose properties depend more on the particle’s precise size and shape than on its composition. The discovery and controlled production of quantum dots is the subject of the 2023 Nobel Prize in Chemistry, which was awarded to MIT professor Moungi Bawendi and two others.

“In a conventional quantum dot, the electron is trapped by the electromagnetic potential generated by a macroscopic number of atoms, so its wave function extends to about 10 nanometers, much larger than the typical atomic radius,” Capellaro said. “Similarly, in these nuclear quantum dots, individual neutrons can be captured by nanocrystals with dimensions well beyond the range of nuclear forces and display similar quantized energies.” While these energy jumps give the quantum dots their color , but neutron quantum dots can be used to store quantum information.

Theoretical basis and simulation

This work is based on theoretical calculations and computational simulations. “We analyzed it in two different ways and finally verified it numerically,” Li said. While this effect has never been described before, he says, there’s in principle no reason why it couldn’t have been discovered earlier: “Conceptually, people should have thought about it,” he says, but as far as the team was able to determine, But no one does.

Part of the difficulty in doing the calculations is that the scales involved are very different: the binding energy of a neutron to the quantum dot to which it is attached is about one trillionth of the previously known conditions for a neutron to bind to a small group of nucleons. In this work, the team used an analytical tool called Green’s function to show that the strong force is sufficient to capture neutrons with quantum dots with a minimum radius of 13 nanometers.

The researchers then performed detailed simulations of specific situations, such as using lithium hydride nanocrystals, a material being studied as a possible storage medium for hydrogen. They showed that the binding energy of neutrons to nanocrystals depends on the exact size and shape of the crystals, as well as the nuclear spin polarization of the atomic nuclei compared to the neutrons. They also calculated similar effects on thin films and wires of materials and particles.

Potential Quantum Applications and Challenges

But actually making such neutron molecules in the lab, one of which would require specialized equipment to maintain temperatures within a few thousandths of a kelvin above, Li said absolute zerois something that other researchers with appropriate expertise must undertake.

Li pointed out that “artificial atoms” are composed of combinations of atoms that share the same properties and can behave in many ways like single atoms, and have been used to probe many properties of real atoms. Likewise, he said, these artificial molecules provide “an interesting model system” for studying “interesting quantum mechanical questions that people can think about,” such as whether these neutron molecules have a shell structure that mimics the shell structure of electron atoms. .

“One possible application,” he said, “maybe we can precisely control the state of neutrons. By changing the way quantum dots oscillate, maybe we can shoot neutrons in specific directions. Neutrons are responsible for triggering reactions such as fission and fusion. A powerful tool, but so far controlling single neutrons has been difficult, he said. These new bound states could provide a greater degree of control over single neutrons, which could play a role in the development of new quantum information systems. .

“One idea is to use it to manipulate neutrons, which would then be able to affect other nuclear spins,” Li said. In this sense, he said, neutron molecules can act as intermediaries between the nuclear spins of different atomic nuclei, a property that has been used as the 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 said. “This is a potential application.” He added that this is “completely different from the electromagnetics-based quantum information processing that has dominated so far. So whether it is superconducting qubits or trapped ions or nitrogen vacancy centers, where Most are based on electromagnetic interactions. Instead, in this new system, “we have neutrons and nuclear spins. We’re just now starting to explore what we can do with it.”

Another possible application, he said, is imaging using neutral activation assays. “Neutron imaging complements X-ray imaging because neutrons interact more strongly with light elements,” Li said. It can also be used in materials analysis to provide information not only on the composition of elements, but even on the different isotopes of those elements. “A lot of chemical imaging and spectroscopy don’t tell us about isotopes,” and neutron-based methods can do that, he said.

Reference: “μeV-deep neutron bound states in nanocrystals” by Hao Tang, Guoqing Wang, Paola Cappellaro and Ju Li, March 15, 2024 ACS Nano.
DOI: 10.1021/acsnano.3c12929

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