Atomic probe for quantum states of the electromagnetic field

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Abstract
When fields are confined to high-Q cavities, their states cannot easily be measured in a direct way. We propose a scheme to measure the state of an unknown field using an atomic probe driven by a reference field. This is a variation of the atomic homodyne detection method devised by Wilkens and Meystre [Phys. Rev. A 43, 3832 (1991)]. We have shown in a previous article [Phys. Rev. A 48, 3168 (1993)] that quantum fluctuations in the driving field cannot be neglected and that these fluctuations limit the resolution of the measurement scheme. In this extension of that work, the atoms are weakly coupled to the driving field although they can couple much more strongly to the cavity field. We show that this enables us to neglect the effects of quantum fluctuations in the driving field over the relevant time scales and allows us to measure more precisely the state of the unknown field by detecting the atoms that exit the cavity in the excited state.