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

TLDR

The paper examines excess noise sources in a practical balanced homodyne detector for Gaussian‑modulated coherent‑state quantum key distribution. The authors refine the realistic model to include detector bandwidth and local‑oscillator fluctuations, and construct a high‑speed balanced homodyne detector operating at telecom wavelengths. They find that the original model can yield insecure keys, but their detector achieves a 104 MHz bandwidth with electronic noise 13 dB below shot noise, enabling secure key rates in the megabits per second range over several kilometres.

Abstract

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 8.5×108 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.