A Sagnac-type Mach-Zehnder interferometer(SMZI) is a very stable instrument with respect to phase drift because all optical paths can share common optical components. We have observed very stable interference fringes in single photon interference experiments with an extremely attenuated laser beam, and with heralded single photons prepared from polarization correlated photon pairs in type-II spontaneous parametric down-conversion. The phase stability is measured to be about ±0.18 rad for the SMZI, in contrast, conventional MZI showed the phase fluctuation of around ±1.02 rad.

We performed experiments on Hong-Ou-Mandel type two-photon interference with frequency entangled photon pairs at 1.5 μm telecommunication band generated through femtosecond pulsed spontaneous parametric down-conversion. Two different angular frequencies   and   were selected using CWDM(coarse wavelength division multiplexing) filters at the output ports of the interferometer. The coincidence counting rates were measured with varying path-length difference between the two interferometer arms to observe the two-photon interference patterns of spatial beating. The obtained visibilities in the net coincidence were close to the theoretical limit of 100%.