Dysprosium-163 has a nuclear spin of 5/2, making its nucleus a six-level quantum system — a qudit rather than a qubit. In a single-molecule magnet like DyPc₂ (dysprosium bis-phthalocyanine), this nuclear spin couples to the electronic magnetic moment through the hyperfine interaction. The electronic moment flips stochastically, producing telegraph noise in the tunneling current measured by a scanning tunneling microscope. The nuclear spin state modifies the flipping statistics.
Chen et al. (arXiv:2603.13047) show that the nuclear spin state can be read directly from the telegraph noise without sweeping any magnetic field. Different nuclear spin projections shift the hyperfine field experienced by the electronic moment, which changes the barrier for electronic spin reversal. This changes the switching rate — the statistical properties of the telegraph noise encode which nuclear spin state the nucleus occupies.
The nuclear spin relaxation times exceed minutes at 35 millikelvin, making the nuclear states stable enough to serve as quantum memory. The authors drive nuclear spin transitions with radiofrequency fields and detect the resulting nuclear magnetic resonance through changes in the tunneling current statistics. The Kondo peaks in the tunneling spectrum split by the hyperfine interaction, and the conductance near these split peaks depends on the nuclear spin state — providing a complementary readout pathway.
The method bypasses the standard approach to nuclear spin detection in molecular magnets, which requires sweeping the external magnetic field through quantum tunneling resonances and detecting steps in the magnetization. That approach is slow and perturbative — the field sweep itself can change the nuclear state you're trying to read. The telegraph noise readout is passive: the electronic moment flips on its own, and you just listen.