🔬 applied physics

A Coherent Neutron-Nucleus Hybrid System Mediated by the Strong Interaction

This paper proposes a coherent neutron-nucleus hybrid system mediated by the strong interaction, where polarized helium-3 nuclei in a spin superposition transfer their nuclear energy phase to incident neutrons, resulting in heterodyne signals at sum or difference frequencies depending on the relative helicity of the coupled spin-energy states.

Original authors: Niels Geerits, Kylie A. Dickerson, Earl Babcock, Andreas Dvorak, Florian Engel, Konstantin Kholostov, Vladimir Ossovyi, Zahir Salhi, Clemens Trunner, Stephan Sponar

Published 2026-10-30

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CC BY 4.0

Original authors: Niels Geerits, Kylie A. Dickerson, Earl Babcock, Andreas Dvorak, Florian Engel, Konstantin Kholostov, Vladimir Ossovyi, Zahir Salhi, Clemens Trunner, Stephan Sponar

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, scientists have known that the tiny particles inside an atom, such as neutrons, behave like waves as well as like solid objects. This wave-like nature allows them to carry information about their journey in a way that is invisible to the naked eye but detectable with sensitive instruments. In the world of quantum physics, these particles can exist in a state of superposition, meaning they can hold two different properties at once, such as spinning in two directions simultaneously. Researchers have long used these properties to probe the hidden structures of materials, but a new experiment has taken this a step further by linking the quantum state of a free-floating neutron directly to the quantum state of a nucleus inside a gas. This work explores whether the internal rhythm of an atomic nucleus can be imprinted onto a passing neutron, creating a hybrid system where the two distinct particles share a single, synchronized quantum phase.

The researchers, working at a nuclear reactor in Austria, set out to test this connection using helium-3, a specific isotope of helium gas. They began by preparing a large group of helium-3 nuclei, aligning their spins and then placing them in a magnetic field. This field caused the nuclei to precess, or wobble, at a specific frequency, much like a spinning top slowing down on a table. The team then fired a stream of free neutrons through this gas. As the neutrons passed through, they interacted with the helium nuclei not through magnetism, but through the strong nuclear force, which is the powerful glue that holds atomic nuclei together. The experiment showed that the neutrons did not just pass through unaffected; instead, they absorbed the rhythmic phase information of the wobbling nuclei. When the researchers measured the intensity of the neutrons after they exited the gas, they saw a distinct pattern of rising and falling signals, known as a beat, that matched the exact frequency of the helium nuclei's wobble. This proved that the energy and phase structure of the atomic nuclei had been transferred directly to the neutrons.

To push the experiment further, the team introduced a second layer of complexity by manipulating the neutrons before they even reached the gas. They used a device that could flip the spin of the neutrons and give them a specific energy kick, putting the neutrons into their own state of superposition. When these prepared neutrons passed through the wobbling helium gas, the two different rhythms—the one from the gas and the one from the neutron preparation device—interfered with each other. The result was a new signal that vibrated at a frequency equal to the sum or the difference of the two original frequencies, depending on how the spins were aligned. This phenomenon, known as heterodyning, demonstrated that the neutron was carrying a combined memory of both its own preparation and the state of the nuclei it had just passed through. The researchers observed these beats clearly, with the signal oscillating at frequencies such as 5 kilohertz or 25 kilohertz, depending on the settings they chose.

The experiment relied on a unique operating mode of the TRIGA reactor in Vienna, which can deliver a massive burst of neutrons in a very short window of time. Because the quantum connection between the neutrons and the nuclei only lasts for a few milliseconds before the delicate state fades away, the researchers needed a high density of particles to get a clear reading. By using these short, intense pulses, they were able to gather enough data to see the oscillations clearly, whereas a steady, continuous stream of neutrons would have been too sparse to detect the effect. The team measured how long this connection lasted, finding that the coherence of the helium nuclei remained stable for about 32 milliseconds, a significant amount of time in the quantum world. This duration allowed them to map out the decay of the signal and confirm that the effect was real and reproducible.

The findings suggest that the strong nuclear force acts as a bridge, mapping the quantum state of a nucleus onto the energy and spin of a passing neutron. The researchers argue that the neutron effectively becomes a partial encoding of the state inhabited by the individual nuclei it encounters. This is not merely a theoretical possibility but a measured reality in their setup, where the neutron acts as a probe that has been physically altered by the quantum state of the sample. The work establishes a new type of hybrid system where two independently controlled parts of a quantum system—the free neutron and the polarized gas—can exchange phase and energy information. This opens a path for using free neutrons to detect and study the quantum mechanical properties of nuclear spins in ways that were previously difficult to achieve, offering a fresh tool for exploring the fundamental nature of matter.

Technical Summary: A Coherent Neutron-Nucleus Hybrid System Mediated by the Strong Interaction

Problem and Motivation
While the quantum mechanical nature of neutrons has been extensively characterized through interferometry, entanglement demonstrations, and Leggett-Garg inequality tests, and while polarized nuclei have been shown to exhibit quantum spin statistics, the direct coupling of a coherent ensemble of polarized nuclei with free neutrons via the strong interaction remains an underexplored regime. Previous work has utilized polarized $^3$ He for neutron beam polarization and spin-dependent scattering, and Abragam demonstrated resonant spin flips in free neutrons driven by precessing proton spins. However, a system where the phase and energy structure of a nuclear spin ensemble is coherently mapped onto the energy degree of freedom of a free neutron has not been experimentally realized. This paper investigates such a hybrid system, aiming to demonstrate that the spin-dependent strong interaction can transfer nuclear spin phases to free neutrons, creating a coherent neutron-nucleus hybrid state.

Methodology
The experiments were conducted at the 250 kW TRIGA reactor at the TU Wien Atominstitut, utilizing a unique pulsed operation mode (250 MW peak power for 50 ms) to overcome flux limitations inherent in continuous operation. The setup involved two primary configurations:

Neutron-Nucleus Coupling: A coherent ensemble of polarized $^3$ He nuclei was prepared in a spin superposition state using Spin-Exchange Optical Pumping (SEOP). A holding magnetic field ( $B_0$ ) induced Zeeman splitting, causing the nuclear spins to precess. Free neutrons, polarized via supermirror reflectors, were passed through the $^3$ He cell. The interaction was modeled as a time-dependent potential where the nuclear polarization acts as a rotating field, coupling the neutron's spin to its energy.
Heterodyne Interference: In a second configuration, a resonant Radio Frequency (RF) flipper was introduced to prepare the incident neutrons in a spin-energy superposition state. This allowed the RF-induced energy phase to interfere with the phase acquired from the nuclear interaction.

Theoretical modeling treated the interaction using the Schrödinger equation with a potential comprising both the magnetic Zeeman term and the spin-dependent strong interaction term. The analysis predicted that the nuclear potential would drive spin-dependent energy transitions, imprinting the nuclear Zeeman frequency onto the neutron wavefunction.

Key Results
The experimental data confirmed the theoretical predictions of coherent phase and energy transfer:

Nuclear Zeeman Frequency Transfer: In the first configuration, the nuclear precession frequency was set to 7.75 kHz, and the neutron intensity exhibited clear beating at this frequency. Fast Fourier Transforms (FFT) of the raw neutron counts from separate measurements confirmed peaks at 6 kHz, 7.75 kHz, and 10 kHz corresponding to the different holding fields applied in distinct experimental runs.
Phase Control and Coherence: By flipping the nuclear spin by $\pi$ prior to precession, the beat signal was observed to shift by $\pi$ , confirming phase control. Analysis of the decay of the beat amplitude over time yielded a transverse nuclear coherence time ( $T_2$ ) of $32.2 \pm 2$ ms.
Heterodyne Mixing: In the second configuration, where neutrons were prepared in a spin-energy superposition via an RF flipper, the system exhibited heterodyne behavior. The observed beat frequencies corresponded to the sum or difference of the RF frequency and the nuclear Larmor frequency, depending on the relative helicity (direction) of the holding fields. For instance, with a 10 kHz nuclear frequency and a 15 kHz RF frequency, beats were observed at 5 kHz (difference) or 25 kHz (sum) based on field alignment.

Key Contributions
The paper establishes a neutron-nucleus hybrid system where:

The strong interaction maps the phase and energy structure of a polarized nuclear ensemble ( $^3$ He) directly onto the energy degree of freedom of free neutrons.
The nuclear Zeeman splitting is transferred to the neutron, creating an intensity beat at the nuclear Larmor frequency.
When neutrons are prepared in a superposition state, the nuclear and RF phases coherently heterodyne, producing signals at sum or difference frequencies.
The final neutron state encodes the phase and energy structure of the nuclear spin state, effectively acting as a partial encoding of the individual nuclear states.

Significance and Claims
The authors argue that these results demonstrate a coherent hybrid system where phase and energy can be transferred and combined between independently controlled subsystems (the nuclear ensemble and the free neutron). The paper posits that the state produced by the neutron-nucleus spin-spin interaction is of the same nature as that produced by an RF flipper, with the $^3$ He ensemble effectively generating an energy superposition state in the neutron.

The significance of this work lies in establishing a new route for probing coherently prepared nuclear spin states using free neutrons as a quantum probe. The system serves as a test bed for quantum neutron scattering, leveraging the exceptional coherence of helium spins and the isolation of neutrons to study quantum mechanical properties of samples. The authors modestly conclude that the neutron state represents a partial encoding of the nuclear state, without claiming broader applications beyond this specific demonstration of coherent coupling and phase transfer.

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