Polarization attack on continuous-variable quantum key distribution

The shot-noise unit (SNU) is a crucial factor for the practical security of a continuous-variable quantum key distribution system. In the most widely used experimental scheme, the SNU should be calibrated first and acts as a constant during key distribution. However, the practical SNU could be controlled by the eavesdropper, which will differ from the calibrated one. It could open loopholes for the eavesdropper to intercept the secret key. In this paper, we report a quantum hacking method to control the practical SNU by using the limited compensation rate in the polarization compensation phase. Since the compensation is only based on of the polarization measurement results of part of local oscillator pulses, the other unmeasured pulses will leave a loophole for eavesdroppers to control the practical SUN. The simulation and experiment results indicate that the practical SNU can be controlled by the eavesdropper. Thus, the eavesdropper can use the fact that, the practical SNU is no longer equals to the calibrated one to control the excess noise and final key rate.