Nonlinear atomic homodyne detection: A technique to detect macroscopic superpositions in a micromaser

We propose a measurement scheme to detect the multiphoton coherences inherent in the macroscopic quantum superpositions that can be generated in a superconducting micromaser cavity. The scheme is a nonlinear version of a single-atom homodyne detector. It is shown that the ionization probability of a single test atom is given in terms of the Wigner characteristic function of the field.