A balanced homodyne detector for high-rate Gaussian-modulated coherent-state quantum key distribution

We discuss the excess noise contributions of a practical balanced homodyne detector (BHD) in Gaussian-modulated coherent-state (GMCS) quantum key distribution (QKD). We point out that the key generated from the original realistic model of GMCS QKD may not be secure. In our refined realistic model, we take into account excess noise due to the finite bandwidth of the homodyne detector and the fluctuation of the local oscillator (LO). A high-speed BHD suitable for GMCS QKD in the telecommunication wavelength region is built and experimentally tested. The 3 dB bandwidth of the BHD is found to be 104 MHz and its electronic noise level is 13 dB below the shot noise at an LO level of 8.5 x 10(8) photons per pulse. The secure key rate of a GMCS QKD experiment with this homodyne detector is expected to reach Mbits s(-1) over a few kilometers.