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.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Quantum key distribution (QKD) enables secure communication between two remote parties in the presence of a potential adversary. The information-theoretic security is guaranteed by the laws of physics and is therefore unconditional. In the case of QKD based on the Gaussian-modulated coherent-state (GMCS) protocol, the Heisenberg uncertainty principle is used to estimate the maximum amount of information leaked to the eavesdropper based on the amount of excess noise (in excess of the quantum noise limit) in the received signal. It is critically important to determine the amount of noise that is potentially controllable by the eavesdropper: underestimation would compromise security, while overestimation would reduce the secure key rate. An existing model widely used to calculate key rates assumes the detector noise consists only of electronic circuit noise which cannot be controlled by the eavesdropper. We observe that detector noise can be caused by other mechanisms, which can be controlled by the eavesdropper, and consequently poses a potential threat to security.
Main results. We present a quantitative analysis of the excess noise of a GMCS QKD system. We identify two new noise sources and quantify their key rate dependences. Moreover, we develop a high-speed detector suitable for GMCS QKD, with which the key rate may be increased more than tenfold.
Wider implications. We demonstrate that careful examination of noise sources is crucial in QKD to prevent loopholes for eavesdroppers. We also identify essential design issues for building an optical homodyne detector for high-speed QKD systems.