Quantum hacking: Induced-photorefraction attack on a practical continuous-variable quantum key distribution system

We explore a new security loophole in a practical continuous-variable quantum key distribution (CVQKD) system, which is opened by the photorefractive effect of lithium niobate-based (LN-based) modulators. By exploiting this loophole, we propose a quantum hacking strategy, i.e., the induced-photorefraction attack, which utilizes the induced photorefraction on the LN-based modulators to hide the classical intercept-resend attack. Specifically, we show that the induced photorefraction can bias the response curve of the LN-based modulator, which will affect the intensity of the modulated signal. Based on the investigation of the channel parameter estimation under above influence, we further analyze the secret key rate of the practical CVQKD system. The simulation results indicate that the communication parties will overestimate the secret key rate, which reveals that Eve can actively open the above loophole by launching the induced-photorefraction attack to successfully obtain the secret key information. To defend against this attack, we can use a random monitoring scheme for modulation variance to determine this attack, and use an improving optical power limiter to effectively mitigate the irradiation beam. Apart from these countermeasures, we also propose using the Sagnac-based IM to stabilize the practical CVQKD system, which can minimize the above effects.