  A real-time polarization control system employing two nonorthogonal reference
signals multiplexed in either time or wavelength with the data signal is
presented. It is shown, theoretically and experimentally, that complete control
of multiple polarization states can be attained employing polarization
controllers in closed-loop configuration. Experimental results show that
negligible added penalties, corresponding to an average added optical Quantum
Bit Error Rate of 0.044%, can be achieved with response times smaller than 10
ms, without significant introduction of noise counts in the quantum channel.
