  Nuclear spins of noble gases feature extremely long coherence times but are
inaccessible to optical photons. Here we realize a coherent interface between
light and noble-gas spins that is mediated by alkali atoms. We demonstrate the
optical excitation of the noble-gas spins and observe the coherent back-action
on the light in the form of high-contrast two-photon spectra. We report on a
record two-photon linewidth of 5$\pm$0.7 mHz (millihertz) above
room-temperature, corresponding to a one-minute coherence time. This experiment
provides a demonstration of coherent bi-directional coupling between light and
noble-gas spins, rendering their long-lived spin coherence accessible for
manipulations in the optical domain.
