Coherent pairs probe disorder-suppressed quantum dynamics in a random dipolar magnet*

Quantum information processing relies on the persistent coherence of quantum states and holds great potential for application, ranging from computation to communication and metrology. However, uncontrolled interactions among quantum bits (qubits) and with their environment lead to decoherence, i.e., the loss of quantum information ^[1,2]. Protecting coherence is generally difficult, particularly in quantum systems with magnetic degrees of freedom which directly couple to electron spin fluctuations. Though, many-body localization (MBL) ^[3,4] and its precursors ^[5-7] have been predicted to suppress relaxation processes and noise in disordered magnets and, thus, to enhance coherence.

I present such a noise suppression with a concurrent increase of coherence times by more than to two orders of magnitude beyond the time scale set by random dipolar interactions in the rare-earth magnet LiY_1-xTb_xF₄. Using Hahn-echo pulse sequences, our measurements find that pairs of atypically close terbium ions host entangled excited states with strongly enhanced coherence times owing to a clock state transition in the presence of suppressed noise and scarce decay channels. These pairs are identified in the frequency domain via coherent driving of Rabi oscillations whose frequency reveals the composite nature of the coupled two-level systems. Exploiting multiple levels of protection for coherence of pair states, we use them as sensitive probes to unveil the heterogeneous dynamics of the interacting quantum magnet.