A two-way actively stabilized QKD system that utilizes control signals and
quantum signals is disclosed. Because the quantum signals do not traverse
the same optical path through the system, signal collisions in the phase
modulator are avoided. This allows the system to have a higher
transmission rate than a two-way system in which the quantum signals
traverse the same optical path. Also, the active stabilization process,
which is based on maintaining a fixed relationship between an intensity
ratio of interfered control signals, is greatly simplified by having the
interferometer loops located all in one QKD station.