  Polarization-encoded free-space Quantum Communication requires a quantum
state source featuring fast polarization modulation, long-term stability and a
low intrinsic error rate. Here we present a source based on a Sagnac
interferometer and composed of polarization maintaining fibers, a fiber
polarization beam splitter and an electro-optic phase modulator. The system
generates predetermined polarization states with a fixed reference frame in
free-space that does not require calibration neither at the transmitter nor at
the receiver. In this way we achieve long-term stability and low error rates. A
proof-of-concept experiment is also reported, demonstrating a Quantum Bit Error
Rate lower than 0.2% for several hours without any active recalibration of the
devices.
