Quantum optics with long-lived optically inaccessible spins

The nuclear spins of noble gases are isolated from the environment exceptionally well and can maintain coherence for hours. Unfortunately, these spins are not accessible to light in the optical domain. Therefore, as opposed to optically-accessible alkali-metal spins employed in quantum optics and metrology, the (potential) quantum qualities of noble-gas spins have been beyond reach and largely ignored. We show that thermal spin-exchange collisions between noble-gas and alkali-metal spins form a quantum interface between them [1,2]. Despite their stochastic nature, these weak collisions accumulate to a deterministic, efficient, and controllable coupling between the collective spins of the two ensembles. The interface paves the way to employing noble-gas spins in the quantum domain, for example, as an optical quantum memory with hour-long lifetimes [3,4]. In experiments, we realize the strong coupling of potassium to helium-3 spins and witness the periodic exchange of their spin coherence [5]. We then introduce light fields and demonstrate the efficient bi-directional optical interface to helium-3 [6]. We discuss the prospects for generating long-lived entanglement between distant noble-gas ensembles [7].