  An ensemble of noble-gas nuclear spins is a unique quantum system that could
maintain coherence for many hours at room temperature and above, owing to
exceptional isolation from the environment. This isolation, however, is a mixed
blessing, limiting the ability of these ensembles to interface with other
quantum systems coherently. Here we show that spin-exchange collisions with
alkali-metal atoms render a quantum interface for noble-gas spins without
impeding their long coherence times. We formulate the many-body theory of the
hybrid system and reveal a collective mechanism that strongly couples the
macroscopic quantum states of the two spin ensembles. Despite their stochastic
and random nature, weak collisions enable entanglement and reversible exchange
of nonclassical excitations in an efficient, controllable, and deterministic
process. With recent experiments now entering the strong-coupling regime, this
interface paves the way towards realizing hour-long quantum memories and
entanglement at room-temperature.
