  Quantum Key Distribution (QKD) allows distant parties to exchange
cryptographic keys with unconditional security by encoding information on the
degrees of freedom of photons. Polarization encoding has been extensively used
in QKD implementations along free-space, optical fiber and satellite-based
links. However, the polarization encoders used in such implementations are
unstable, expensive, complex and can even exhibit side-channels that undermine
the security of the implemented protocol. Here we propose a self-compensating
polarization encoder based on a Lithium Niobate phase modulator inside a Sagnac
interferometer and implement it using only standard telecommunication
commercial off-the-shelves components (COTS). Our polarization encoder combines
a simple design and high stability reaching an intrinsic quantum bit error rate
as low as 0.2%. Since realization is possible from the 800 nm to the 1550 nm
band by using COTS, our polarization modulator is a promising solution for
free-space, fiber and satellite-based QKD.
