We present a discretely modulated continuous-variable quantum key distribution system in free space by using strong coherent states. The amplitude noise in the laser source is suppressed to the shot-noise limit by using a mode cleaner combined with a frequency shift technique. Also, it is proven that the phase noise in the source has no impact on the final secret key rate. In order to increase the encoding rate, we use broadband homodyne detectors and the no-switching protocol. In a realistic model, we establish a secret key rate of $46.8$ kbits/s against collective attacks at an encoding rate of $10$ MHz for a $90\%$ channel loss when the modulation variance is optimal.

• Received 30 December 2009

DOI:https://doi.org/10.1103/PhysRevA.82.022317