  We present a stable and deterministic quantum key distribution (QKD) system
based on differential phase shift. With three cascaded Mach-Zehnder
interferometers with different arm-length differences for creating key, its key
creation efficiency can be improved to be 7/8, more than other systems. Any
birefringence effects and polarization-dependent losses in the long-distance
fiber are automatically compensated with a Faraday mirror. Added an
eavesdropping check, this system is more secure than some other
phase-coding-based QKD systems. Moreover, the classical information exchanged
is reduces largely and the modulation of phase shifts is simplified. All these
features make this QKD system more convenient than others in a practical
application.
