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Squeezing of Spin Waves in Atomic Ensembles

Ben Baragiola, Center for Quantum Information and Control, University of New Mexico

(Session 10 : Saturday from 2:00 - 2:30)

Abstract. Squeezing the collective spin of an atomic ensemble via QND measurement is based on the interaction between a cloud of atoms and a laser probe. When the shot noise resolution of the laser probe is below the projection noise fluctuations of the atoms, the resulting backaction can reduce the uncertainty for a collective atomic observable. Most current models of this process rely on idealized one-dimensional plane wave approximations of the underlying light-matter interaction, which are not appropriate for describing a real system consisting of a cigar-shaped cold atomic cloud in dipole trap interacting with a probe laser. We extend such models from first principles to include spatial dependence of both the light and of the atomic ensemble and apply it to QND spin squeezing for large-spin alkali atoms. The model includes spin waves, diffraction, paraxial modes, and optical pumping, derived by a full master equation description. We find that to optimally mode-match for spin squeezing one must consider not only collective scattering into the forward modes of the field, but also the effects of local decoherence from spatially-varying diffuse photon scattering. Surprisingly, in certain circumstances such local decoherence can be less destructive to spin squeezing than previously thought from phenomenological models.