Quantum optics with noble-gas spins

Noble-gas nuclear spins are extremely isolated from the environment and can maintain spin 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. 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 the long coherence time of noble-gas spin in the quantum domain. We present a quantum treatment of the stochastic collisional process and analyze the prospects for realizing non-classical states and quantum memories with hour-long lifetimes. In experiments, we realize the strong coupling of potassium to helium-3 spins and witness their periodic exchange of spin coherence. We then introduce light fields and demonstrate the efficient optical interface to helium-3. We discuss the prospect for generating long-lived entanglement between distant noble-gas ensembles.