Squeezing the angular momentum of an ensemble of complex multi-level atoms

Squeezing of collective atomic spins has been shown to improve the sensitivity of atomic clocks and magnetometers beyond the standard quantum limit. In typical experiments the atoms act as pseudo-spin-1/2 particles and squeezing results solely from spin-spin correlations. Here we implement a high quality quantum nondemolition (QND) measurement and generate u003e 4 dB of metrological squeezing for a physical angular momentum made from the large internal spins of complex multilevel atoms. Squeezing by QND measurement has the potential for significant enhancement though combined internal-collective spin control, and sets the stage for experiments on quantum feedback, deterministic squeezing, and closed-loop magnetometry.