April 13, 2024

Study shows that neutrons can bind to quantum dots

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MIT researchers have discovered “neutronic” molecules, in which neutrons can stick to quantum dots, held only by strong force. The discovery could lead to new tools for probing the properties of materials at the quantum level and exploring new types of quantum information processing devices. Here, the red item represents a bound neutron, the sphere is a hydride nanoparticle, and the yellow field represents a neutron wave function. Credit: Massachusetts Institute of Technology

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MIT researchers have discovered “neutronic” molecules, in which neutrons can stick to quantum dots, held only by strong force. The discovery could lead to new tools for probing the properties of materials at the quantum level and exploring new types of quantum information processing devices. Here, the red item represents a bound neutron, the sphere is a hydride nanoparticle, and the yellow field represents a neutron wave function. Credit: Massachusetts Institute of Technology

Neutrons are subatomic particles that do not have an electrical charge, unlike protons and electrons. This means that although the electromagnetic force is responsible for most interactions between radiation and materials, neutrons are essentially immune to this force.

Instead, neutrons are held together inside an atom’s nucleus only by something called the strong force, one of the four fundamental forces of nature. As its name suggests, the force is indeed very strong, but only over a very short distance – it diminishes so quickly that it is negligible beyond 1/10,000 the size of an atom.

But now, MIT researchers have discovered that it is possible to make neutrons stick to particles called quantum dots, which are made up of tens of thousands of atomic nuclei, held there only by strong force.

The new discovery could 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 types of quantum information processing devices.

The work was published this week in the magazine ACS Nano in a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT professors Ju Li and Paola Cappellaro in the Department of Nuclear Science and Engineering.

Neutrons are widely used to probe properties of materials 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 internal structure and the dynamics of the material.

But until this new work, no one thought these neutrons could actually stick to the materials they were probing. “The fact that [the neutrons] “We were surprised that this exists and that no one had talked about it before, among the experts who had checked,” he says. he.

The reason this new discovery is so surprising, explains Li, is because neutrons do not interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force “are generally not important for materials,” he says. “Almost everything is electromagnetic interaction, but in this case, as the neutron has no charge, the interaction here is through the strong interaction, and we know that it is very short-range. to the power of minus 15″, 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 that we have thousands of nuclei in this neutronic quantum dot, and this is able to stabilize these bound states, which have much more diffuse wave functions over tens of nanometers. These neutronic bound states in a quantum dot are in truth quite similar to Thomson’s plum pudding model of an atom after his discovery of the electron.”

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

These neutron molecules are made of 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 was 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 captured by the electromagnetic potential created by a macroscopic number of atoms, so its wave function 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 far beyond the reach of the nuclear force, and exhibit similar quantized energies.” Although these energy jumps give quantum dots their colors, neutronic quantum dots could be used to store quantum information.

This work is based on theoretical calculations and computer simulations. “We did this analytically in two different ways, and eventually we also verified it numerically,” says Li. Although the effect has never been described before, he says, there is in principle no reason why it couldn’t have been found much earlier: “Conceptually, people should have already thought of this,” he says, but as far as the team has been able to determine, no one has.

Part of the difficulty in doing the calculations is the very different scales involved: the binding energy of a neutron to the quantum dots to which they were bound is about a trillionth of that under previously known conditions, where the neutron is bound to a small group of cores. . 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 carried out detailed simulations of specific cases, such as the use of a lithium hydride nanocrystal, a material that is being studied as a possible means of storing hydrogen. They showed that the binding energy of the neutrons to the nanocrystal depends on the exact dimensions and shape of the crystal, as well as the polarizations of the nuclear spin of the nuclei compared to that of the neutron. They also calculated similar effects for thin films and strands of the material, as opposed to particles.

But Li says that creating such neutron molecules in the laboratory, 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 experience will have to undertake. .

Li notes that “artificial atoms” made up of sets 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. Likewise, he says, these artificial molecules provide “an interesting model system” that can 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 layer structure. of atoms.

“One possible application,” he says, “is perhaps we can precisely control the state of the neutron. By changing the way the quantum dot oscillates, perhaps we can fire the neutron in a specific direction.” Neutrons are powerful tools for triggering fission and fusion reactions, but until now 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,” says Li. In this sense, he says, the neutron 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 the development of quantum computer systems.

“The nuclear spin is like a stationary qubit, and the neutron is like a flying qubit,” he says. “That’s a potential application.” He adds that this is “very different from quantum information processing based on electromagnetism, which until now is the dominant paradigm. Therefore, regardless of whether they are superconducting qubits or trapped ions or nitrogen vacancy centers, most of them 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 that now.”

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

More information:
Hao Tang et al, μeV-Deep neutron bound states in nanocrystals, ACS Nano (2024). DOI: 10.1021/acsnano.3c12929

Diary information:
ACS Nano

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