  The BB84 and B92 protocols based on polarization coding are usually used in
free-space quantum key distribution. Polarization tracking technique is one of
the key techniques in the satellite quantum key distribution. Because the
photon polarization state will be changed as a result of the satellite
movement, both the transmitter and receiver need to have the ability to track
the polarization orientation variation to accomplish the quantum cryptography
protocols. In this paper, the polarization tracking principles are analyzed
based on Faraday effect and the half-wave plate. The transforms of six photon
polarization states in three conjugative bases are given and the quantum key
coding principles based on the polarization tracking are analyzed for the BB84
and B92 protocols.
