Quantum sensors lift the veil on neutrinos

The existence of neutrinos was first hypothesized by Wolfgang Pauli in 1930 in a desperate attempt to save the fundamental principle of the conservation of energy in beta decay, in which it appeared that energy was lost. It turned out that neutrinos do exist, but their interaction with matter is extremely limited. Studying neutrinos requires very large detection systems like the IceCube Neutrino Observatory in Antarctica, with its cubic kilometer of ice. Today, still, neutrinos’ properties remain mysterious. While the Standard Model of particle physics predicts neutrino masses to be zero, oscillation experiments have shown that they do indeed have mass. Three
types of neutrinos are currently known, and the existence of a fourth, the sterile neutrino, coupled only to gravity, is still merely hypothetical.

The BeEST consortium, led by Kyle G. Leach, was created in 2020 to study neutrinos. LHNB has been involved in the project since its inception, contributing to an innovative “tabletop” experiment to measure the properties of neutrinos indirectly using extremely small sensors (about
the diameter of a human hair). The radioactive beryllium-7 nucleus, which decays by capturing an atomic electron, is implanted in a superconducting tunnel junction—a quantum sensor with exceptionally high energy resolution. The ultra-precise measurement of the nucleus’ recoil energy—a spectrum of just a hundred electronvolts—provides information about the neutrino emitted, as the two particles are quantum-entangled.

Neutrinos, like any quantum particle, cannot be specifically  located. However, there is a probability that one will be present in a certain area of space. Until now, this probability was assumed to correspond to the size of the neutrino’s nucleus. The BeEST experiment yielded a very unexpected observation: As soon as the neutrino is created, it extends over an area thousands of times larger than the nucleus. This finding upends everything
we thought we knew about radioactive decay.

The method in the experiment is so precise that it can also be used to observe the effect of the chemical environment on the nuclear process. One of the BeEST consortium’s first findings shows that, between measurement in a tantalum detector and a theoretical prediction for a nucleus in a vacuum, the probability of beryllium-7 electron capture can vary by a factor of two. The BetaShape code, developed by LNHB over the past fifteen years and now an international reference for the evaluation of beta decay data, was used for the theoretical prediction. The effect of the chemical environment is currently the main scientific bottleneck limiting the accuracy of the physical information that can be extracted from the BeEST experiment. Additional—and complex—theoretical research to model this effect as accurately as possible is in progress with Paul-Antoine
Hervieux, Professor at Strasbourg University.

Projet

• The quantum sensors’ exceptional sensitivity will open up new possibilities for the study of
  the chemical environment’s effect on nuclear processes. A project funded through an ERC
  Synergy Grant currently in progress is focusing on developing a multi-scale model that
  preserves quantum hybridization, from the material (10-3 m) to the nucleus (10-15 m). This
  model would remove the Standard Model physics component from the beryllium-7
  spectrum. While neutrinos cannot be considered dark matter according to the Standard Model, this advance could enable the exploration of a new physics—that of massive neutrinos—potentially putting the particles back in the running as candidates for dark matter. It could also lead to a new approach to predicting the best materials to use to influence the radioactive decay process in a controlled way.

Flagship publication

• This breakthrough was published in the journal Nature: « Direct experimental constraints on the spatial extension of a neutrino wavepacket », J. Smolsky et al.,
  Nature 638, 640-644 (2025), https://www.nature.com/articles/s41586-024-08479-6