  We report a single-photon Mach-Zehnder interferometer stabilized to a phase
precision of 0.05 degrees over 15 hours. To lock the phase, we employ an
auxiliary reference light at a different wavelength than the quantum signal.
The developed phase locking operates continuously, with negligible crosstalk,
and for an arbitrary phase of the quantum signal. Moreover, its performance is
independent of intensity fluctuations of the reference. Since the presented
method can be used in a vast majority of quantum interferometric networks it
can significantly improve phase-sensitive applications in quantum communication
and quantum metrology.
