Figure 2: Noisy Mach–Zehnder interferometer.

Experimental set-up. A heralded single photon with 810 nm central wavelength and 3 nm bandwidth from a down-conversion source is split between two paths using a calcite crystal. Polarizations in individual paths are prepared using optional half-wave plates (HWPs) H₁ and H₂. Noise Λ is introduced using four sets of orientations of HWPs H₃ and H₄ and quarter-wave plates Q₁ and Q₂ mounted on computer-controlled motorized rotation stages. Polarization components of the output state are filtered using a HWP H₅ and a second calcite crystal. Interference is realized in the common-path configuration using HWP H₆ oriented at 45° with respect to the set-up plane and a polarizing beam splitter P with output ports monitored by single-photon detectors D₊ and D₋ . Two other detectors D₀ and D₁ are used to determine the overall count rate. The phase shift is introduced by rotating the second calcite crystal about an axis perpendicular to the set-up plane using a closed loop piezo actuator.