The integration of nanophotonics with ultracold atoms opens the door to new protocols in quantum information processing. Strong entangling interactions between atoms and photons are the key ingredient. Whereas a resonant interaction can lead to the strongest entanglement per atom, this requires special geometries that limit decoherence. Off-resonant dispersive interactions, where a phase shift is associated with the atom-photon interaction, provides an alternative route to strong entanglement. This can be achieved due to the “cooperativity” of a large ensemble of atoms that can be homogeneously trapped in the evanescent field of an optical nanofiber using well-known techniques (see figure above). The optical scattering cross section closely matches the guided beam mode area across the entire length of the nanofiber. Our recent paper pedagogically develops the theory to describe how the light dispersively responds to an ensemble of atoms in the optical nanofiber waveguide platform and how this can yield large cooperativity. As an application of the theory, we study the creation of spin squeezed states for application in improved precision of atomic clocks. With only a few thousand Cesium atoms, a nontrivial squeezed state can be created using an anisotropic property of the nanofiber modes, which is not available or hard to implement in free space. This is a first step towards more general protocols involving the production of nonGaussian atomic states and their interaction with nonclassical light.
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