Stable single-photon interference in a 1 km fiber-optical Mach-Zehnder   interferometer with continuous phase adjustment
  We experimentally demonstrate stable and user-adjustable single-photon interference in a 1 km long fiber- optical Mach-Zehnder interferometer, using an active phase control system with the feedback provided by a classical laser. We are able to continuously tune the single-photon phase difference between the interferometer arms using a phase modulator, which is synchronized with the gate window of the single-photon detectors. The phase control system employs a piezoelectric fiber stretcher to stabilize the phase drift in the interferometer. A single-photon net visibility of 0.97 is obtained, yielding future possibilities for experimental realizations of quantum repeaters in optical fibers, and violation of Bell's inequalities using genuine energy-time entanglement 
G. B. Xavier and J. P. von der Weid
quant-ph physics.optics