Quantum noise reduction close to an optically bistable dark resonance

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Abstract
We show that Λ three-level atoms interacting with two coherent light fields tuned close to two-photon or dark resonance behave as a saturable dispersive nonlinearity. If one of the light fields is allowed to resonate within an optical cavity the quantum noise in a quadrature of the output light can be almost completely suppressed. As no dynamic retuning of the cavity is required to achieve noise reduction and cold atomic media are readily available in traps which conform in essence to the proposed configuration, an experimental realization would seem within reach.