  Polarizations of single-photon pulses have been controlled with long-term
stability of more than 10 hours by using an active feedback technique for
auto-compensation of unpredictable polarization scrambling in long-distance
fiber. Experimental tests of long-term operations in 50, 75 and 100 km fibers
demonstrated that such a single-photon polarization control supported stable
polarization encoding in long-distance fibers to facilitate stable one-way
fiber system for polarization-encoded quantum key distribution, providing
quantum bit error rates below the absolute security threshold.
