  In this paper we describe within the formalism of Quantum Optics (QO) a
double Mach-Zehnder interferometer (MZI). For single photon Fock states this
experimental setup is shown to exhibit a counter-intuitive behavior: for
certain values of the path length difference of the first MZI, the singles
photon-count statistics at the output detectors does not change, whatever the
difference in path length for the second MZI. For simultaneously impinging
light quanta, we show that this setup is able to show the same HOM antibunching
effect previously obtained with a beam splitter. However, by adding substantial
delays in each MZI, we can obtain the same effect even if the "photon wave
packets" do not overlap at the second beam splitter.
