We calculate the first-order interference-fringe visibilities for Mach-Zehnder interferometers in which the input beam splitting has time-varying reflection and transmission coefficients. Particular attention is given to single-photon input states, and it is shown that fringes persist in appropriate conditions even for a beam splitter that alternates between complete transmission and complete reflection. The similar behavior found for input coherent and chaotic light confirms the single-photon nature of first-order interference. The states of excitation of the internal arms of the interferometer are derived and discussed in the context of delayed-choice experiments.

• Received 23 May 1991

DOI:https://doi.org/10.1103/PhysRevA.45.1987