  In practical continuous-variable quantum key distribution system using local
local oscillator (LLO CV-QKD), the phase noise related to coherent detection
and phase-reference pulse intensity that can be locally calibrated at the
receiver side is considered to be trusted noise to improve the performance.
However, if the intensity of the phase-reference pulse is not monitored
precisely in real-time, a security loophole will be opened for the eavesdropper
to launch attacks. To this end, we propose two practical phase-reference pulse
intensity attack strategies, where Eve can reduce the trusted phase noise by
manipulating the intensity of the phase-reference pulse, thereby hiding her
attack on the signal pulse if the total excess noise remains unchanged. The
performance of the LLO CV-QKD system under these attacks has been analyzed. We
show that precisely monitoring the intensity of the phase-reference pulse in
real-time is an essential countermeasure to prevent the proposed attacks.
Moreover, an intensity-monitoring scheme for the phase-reference pulse is
proposed to strengthen the security of the practical LLO CV-QKD system and make
the trusted phase noise model more robust.

Quantum Physics