Quantum interface for noble-gas spins.

Macroscopic quantum systems typically suffer from rapid loss of coherence via coupling to the environment. An ensemble of noble-gas nuclear spins is a unique isolated system that could maintain its coherence for many hours at room temperature and above. However, this isolation is a mixed blessing, impeding the coherent interfacing of noble-gas spins with other quantum systems. Here we show that spin-exchange collisions with alkali-metal atoms provide for such a quantum interface. We formulate the many-body theory of the hybrid system and reveal a collective mechanism that can strongly couple the macroscopic quantum states of the two spin gases. Despite their stochastic and random nature, these weak collisions enable entanglement and reversible exchange of non-classical states between the ensembles in an efficient, controllable, and deterministic manner. We outline feasible experimental parameters for reaching the strong-coupling regime, paving the way towards hour-long quantum memories and entanglement at room-temperature.