Nuclear Spin Squeezing in Helium-3 by Continuous Quantum Nondemolition Measurement

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