Abstract: We show that the technique of Ramsey interferometry may be used to completely determine the quantum state of an electromagnetic field in a cavity. Our method, which allows the direct measurement of the Wigner function of the field, is a useful tool for probing the quantum-classical transition and, in particular, the decoherence between coherent superpositions of distinguishable coherent states of the electromagnetic field in the cavity. We also show that the realization of a controlled-not gate within framework of cavity QED is a special case of our proposal, and leads to the measurement of a negative value for the Wigner function of an electromagnetic field.!19
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