Emergence of highly coherent quantum subsystems of a noisy and dense spin system

Quantum sensors and qubits are usually two-level systems (TLS), the quantum analogs of classical bits which assume binary values '0' or '1'. They are useful to the extent to which they can persist in quantum superpositions of '0' and '1' in real environments. However, such TLS are never alone in real materials and devices, and couplings to other degrees of freedom limit the lifetimes - called decoherence times - of the superposition states. Decoherence occurs via two major routes - excitation hopping and fluctuating electromagnetic fields. Common mitigation strategies are based on material improvements, exploitation of clock states which couple only to second rather than first order to external perturbations, and reduction of interactions via extreme dilution of pure materials made from isotopes selected to minimize noise from nuclear spins. We demonstrate that for a dense TLS network in a noisy nuclear spin bath, we can take advantage of interactions to pass from hopping to fluctuation dominance, increasing decoherence times by almost three orders of magnitude. In the dilute rare-earth insulator LiY1-xTbxF4, Tb ions realize TLS characterized by a 30GHz splitting and readily implemented clock states. Dipolar interactions lead to coherent, localized pairs of Tb ions, that decohere due to fluctuating quantum mechanical ring-exchange interaction, sensing the slow dynamics of the surrounding, nearly localized Tb spins. The hopping and fluctuation regimes are sharply distinguished by their Rabi oscillations and the invisible vs. strong effect of classic 'error correcting' microwave pulse sequences. Laying open the decoherence mechanisms at play in a dense, disordered and noisy network of interacting TLS, our work expands the search space for quantum sensors and qubits to include clusters in dense, disordered materials, that can be explored for localization effects.