  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
