Quantum cryptography using pulsed homodyne detection

We report an experimental quantum key distribution that utilizes pulsed homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, homodyne detection allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. Our prototype system works at $1.55\mu \mathrm{m}$ wavelength and the quantum channel is a 1-km standard optical fiber. The probability distribution of the measured electric-field amplitude has a Gaussian shape. The effect of experimental imperfections such as optical loss and detector noise can be parametrized by the variance and the mean value of the Gaussian distribution.