Neutrons are subatomic particles that, unlike protons and electrons, do not carry an electric charge. This means that electromagnetic forces are responsible for most interactions between radiation and matter, but neutrons are essentially immune to those forces.
Instead, neutrons are held together inside the atomic nucleus only by the strong force, one of the four fundamental forces of nature. As the name suggests, the force is indeed very powerful, but only occurs at very close ranges. It decays negligibly quickly beyond 1/10,000th the size of an atom. But now MIT researchers have discovered that neutrons can actually stick to particles called quantum dots, which are made up of tens of thousands of atomic nuclei held together only by strong forces.
The new discovery could lead to new tools useful for probing fundamental properties of materials at the quantum level, including those arising from strong forces, and for exploring new classes of quantum information processing devices. The work was reported in the journal this week. ACS NanoFrom a paper by MIT graduate students Hao Tang and Guoqing Wang and MIT Department of Nuclear Science and Engineering professors Ju Li and Paola Cappellaro.
Neutrons are widely used to probe material properties using a method called neutron scattering. In this method, a neutron beam is focused on a sample and the internal structure and dynamics of the material can be revealed by detecting neutrons bouncing off the material's atoms.
But until this new study, no one thought that these neutrons could actually stick to the material being explored. “Actually that [the neutrons] “You can be trapped by materials and no one seems to know about it,” says Li, who is also a professor of materials science and engineering. “We were surprised that this existed and that no one had ever talked about this before among our experts. We checked,” she said.
What makes this new discovery so surprising, Li explains, is that 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 an electromagnetic interaction, but in this case the neutrons don’t have a charge, so the interaction here is through a strong interaction, and we know that it’s very short-ranged. It’s effective in the range of 10, minus 15 to the power. “, that is, 1/1000th of a meter.
“It’s very small, but very powerful,” he says of this force that holds atomic nuclei together. “But what’s interesting is that these neutron quantum dots have thousands of nuclei, and this can stabilize these bound states, which have much more diffuse wavefunctions in the tens of nanometers. [billionths of a meter]. “These neutron-bound states of quantum dots are actually very similar to the plum pudding model for atoms after Thomson discovered the electron.”
This was so unexpected, Li calls it “a very crazy solution to a quantum mechanics problem.” The team calls the newly discovered state an artificial “neutron molecule.”
These neutron molecules are made of quantum dots, which are tiny crystal particles. Quantum dots are collections of atoms so small that their properties depend more on the exact size and shape of the particle than 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, electrons are trapped by electromagnetic potentials generated by a macroscopic number of atoms, so that their wavefunction extends to about 10 nanometers, much larger than a typical atomic radius,” Cappellaro said. Likewise, in these nuclear quantum dots, a single neutron can be trapped in a nanocrystal with a size well beyond the range of nuclear forces and exhibit similar quantized energies. These energy jumps give quantum dots their color, but neutron quantum dots can 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 verified it numerically as well,” Li said. He says the effect has never been described before, but there's no reason in principle why it shouldn't have been discovered sooner. “Conceptually, people should have already thought about it,” he says. It could have been decided, but no one could decide.
One of the difficulties in performing calculations is the wide variety of scales involved. The binding energy of a neutron and a quantum dot is about one trillionth of the previously known conditions under which neutrons are bound to a small group of nuclei. . For this study, the research team used an analysis tool called Green's function to demonstrate 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 cases, such as the use of lithium hydride nanocrystals, a material under investigation, as a hydrogen storage medium. They showed that the binding energy of a neutron to a nanocrystal depends not only on the exact size and shape of the crystal, but also on the nuclear spin polarization of the nucleus relative to the neutron. They also calculated similar effects for thin films and wires of the material, as opposed to particles.
But Li says that actually generating such neutron molecules in a laboratory, which requires special equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, will have to be done by other researchers with the appropriate expertise. .
Li points out that “artificial atoms,” made up of collections of atoms that share properties and can behave in different ways like single atoms, 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 to think about,” such as whether neutron molecules might have shell structures that mimic electron shell structures. of atoms.
“One possible application is that we could precisely control the state of the neutrons. By changing the way the quantum dots vibrate, we might be able to fire neutrons in specific directions,” he says. Neutrons are powerful tools for triggering nuclear fission and fusion reactions, but until now individual neutrons have been difficult to control. These new bonded states could provide a much higher level 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 this to manipulate neutrons, so they can affect other nuclear spins,” Li says. In that sense, he says, neutron molecules can act as intermediaries between the nuclear spins of individual nuclei, a property that is already being used as basic storage units, or qubits, 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 one potential application.” He added that this is “significantly different from electromagnetism-based quantum information processing, which is the dominant paradigm to date.” So, regardless of whether they are superconducting qubits, trapped ions or nitrogen vacancy centers, most of them are based on electromagnetic interactions. ” Instead, in this new system, “we have neutrons and nuclear spin. “We are now starting to explore what we can do with it.”
Another possible application is a type of imaging using neutral activation analysis, he says. “Neutron imaging complements X-ray imaging because neutrons interact much more strongly with light elements,” Li says. It can also be used for materials analysis, which can provide information not only about elemental composition, but also about the various isotopes of that element. “A lot of chemical imaging and spectroscopy don’t tell you about isotopes, but neutron-based methods can,” he says.
This research was supported by the Office of Naval Research.