Bringing noble-gas spins into the light

In quantum science, we often encounter the tension between elongating the coherence time of a system and retaining the ability to control and interact with it. An extreme example is the nuclear spin of noble gases, which is isolated from the environment by the complete electronic shells. In our lab, the spins of a helium-3 gas maintain coherence for up to two hours. Unfortunately, these spins are not accessible to light in the optical domain, and their (potential) quantum qualities have been beyond reach and largely overlooked. We establish that thermal spin-exchange collisions between noble-gas atoms and alkali-metal atoms form a quantum interface between them. These weak collisions, despite their stochastic nature, accumulate to a deterministic, efficient, and controllable coupling between the collective spins of the two gases. In experiments, we realize the strong coupling between potassium and helium-3 spins and, by coupling light to the potassium spins, demonstrate an efficient, two-way, optical interface to the helium-3 spins. The interface paves the way to employing noble-gas spins in the quantum domain, and we discuss prospects for quantum memories and entanglement of distant noble-gas ensembles with hour-long lifetimes.