  We propose a technique to control the macroscopic collective nuclear spin of
a Helium-3 vapor in the quantum regime using light. The scheme relies on
metastability exchange collisions to mediate interactions between optically
accessible metastable states and the ground-state nuclear spin, giving rise to
an effective nuclear spin-light quantum nondemolition interaction of the
Faraday form. Our technique enables measurement-based quantum control of
nuclear spins, such as the preparation of spin-squeezed states. This, combined
with the day-long coherence time of nuclear spin states in Helium-3, opens the
possibility for a number of applications in quantum technology.
