Unlike protons and electrons, neutrons have no electric charge. The strong force at very close range holds them together inside an atom’s nucleus. The charge drops off so fast that it is negligible beyond 1/10,000 the size of an atom.
MIT scientists have now discovered that neutrons can bind to quantum dots. The study has applications in determining the fundamental properties of materials at the quantum level and exploring new kinds of quantum information processing devices.
Neutrons are commonly used in neutron scattering, a technique in which a beam of neutrons is focused on a sample. The neutrons that bounce off the material’s atoms can be detected to reveal the material’s internal structure and dynamics.
Until now, no one thought neutrons could stick to quantum dots. This new study surprised scientists, as none of the experts we checked with had discussed it before.
MIT professor Ju Li said, “This new finding is so surprising because neutrons don’t interact with electromagnetic forces. Of the four fundamental forces, gravity and the weak force are generally unimportant for materials. Almost everything is electromagnetic interaction, but in this case, since the neutron doesn’t have a charge, the interaction here is through the strong interaction, which we know is very short-range. It is effective at 10 to the minus 15 power, or one quadrillionth, of a meter.”
“It’s tiny, 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 akin to Thomson’s plum pudding model of an atom after his discovery of the electron.”
“It is a pretty crazy solution to a quantum mechanical problem.”
Scientists dubbed the newly discovered state an artificial “neutronic molecule.”
These neutronic molecules are composed of quantum dots, which are minuscule crystalline particles made up of groups of atoms so tiny that their precise size and form determine more of their characteristics than their makeup.
The electromagnetic potential traps electrons in conventional quantum dots. Its wavefunction extends to about 10 nanometers, much larger than a typical atomic radius. Likewise, 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.
The neutronic quantum dots have the potential to store quantum information even though these energy jumps give quantum dots their colors.
For this work, scientists performed theoretical calculations and computational simulations. They did this analytically in two different ways and eventually also verified it numerically.
This effect was not observed before.
The challenge during the computations was the inclusion of different scales. A neutron’s binding energy to the quantum dots it was attaching to is around a trillionth of what was previously known when the neutron was coupled to a tiny number of nuclei. The team’s method for proving that the vital force was adequate to trap neutrons with a quantum dot with a minimum radius of 13 nanometers involved using an analytical tool known as Green’s function.
Scientists next simulated specific cases, such as using a lithium hydride nanocrystal. The simulation showed that the binding energy of the neutrons to the nanocrystal is dependent on the exact dimensions and shape of the crystal and the nuclear spin polarizations of the nuclei compared to that of the neutron. Similar effects were also calculated for thin films and wires of the material as opposed to particles.
But Li says that creating such neutronic molecules in the lab, which requires specialized equipment to maintain temperatures in the range of a few thousandths of a Kelvin above absolute zero, is something 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 natural 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 how the quantum dot oscillates, we may shoot the neutron off in a particular direction.” Neutrons are potent tools for triggering fission and fusion reactions, but so far it has been difficult to control individual neutrons. He says 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.
“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, 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, providing 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,” he says the neutron-based method could do so.
Journal Reference:
- Hao Tang, Guoqing Wang, Paola Cappellaro, and Ju Li. μeV-Deep Neutron Bound States in Nanocrystals. ACS Nano. DOI: 10.1021/acsnano.3c12929