Neutrino detection via spin precession of polarized helium-3

Detection of fundamental particles has traditionally relied on rare, discrete events, requiring massive detectors or long exposures to overcome small cross sections. In contrast, quantum sensors have reached record sensitivities via the coherent coupling of a classical field to an ensemble, shifting all atoms’ quantum states uniformly. Here, we propose adopting this paradigm to measure neutrinos via spin interactions with a helium-3 detector. We compute the unitary evolution of a nuclear spin induced by the elastic forward scattering of a neutrino and the cumulative effect of many such subthreshold interactions on the ensemble spin. While the accumulated spin precession grows linearly with event count, we find that the precession rate is largely independent of detector size, pointing to the possibility of compact neutrino detectors using polarized helium-3.