  Quantum measurements that use the entangled photons' polarization to encode
quantum information require calibration and alignment of the measurement bases
between spatially separate observers. Because of the changing birefringence in
optical fibers arising from temperature fluctuations or external mechanical
vibrations, the polarization state at the end of a fiber channel is
unpredictable and time-varying. Polarization tracking and stabilization methods
originally developed for classical optical communications cannot be applied to
polarization-entangled photons, where the separately detected photons are
statistically unpolarized, yet quantum mechanically correlated. We report here
a fast method for automatic alignment and dynamic tracking of the polarization
measurement bases between spatially separated detectors. The system uses the
Nelder-Mead simplex method to minimize the observed coincidence rate between
non-locally measured entangled photon pairs, without relying on classical
wavelength-multiplexed pilot tones or temporally interleaved polarized photons.
Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well
as in a controlled drifting scenario.
