Continuous-variable quantum key distribution with a leakage from state preparation

Abstract

We address side-channel leakage in a trusted preparation station of continuous-variable quantum key distribution with coherent and squeezed states. We consider two different scenarios: multimode Gaussian modulation, directly accessible to an eavesdropper, or side-channel loss of the signal states prior to the modulation stage. We show the negative impact of excessive modulation on both the coherent-and squeezed-state protocols. The impact is more pronounced for squeezed-state protocols and may require optimization of squeezing in the case of noisy quantum channels. Further, we demonstrate that the coherent-state protocol is immune to side-channel signal state leakage prior to modulation, while the squeezed-state protocol is vulnerable to such attacks, becoming more sensitive to the noise in the channel. In the general case of noisy quantum channels the signal squeezing can be optimized to provide best performance of the protocol in the presence of side-channel leakage prior to modulation. Our results demonstrate that leakage from the trusted source in continuous-variable quantum key distribution should not be underestimated and squeezing optimization is needed to overcome coherent state protocols.

Advances in quantum cryptography

“…Derkach et al suggest some countermeasures to the sender side-channel based on manipulation of the input vacuum state to the beamsplitter, and to the receiver side-channel based on measuring the output of the coupled noise. They then expanded on their earlier work [563], considering two types of side-channel leakage: leakage after modulation of the signal state and leakage prior to modulation, but after squeezing of the signal state (in the squeezed state protocol). They allowed multiple leakage modes from each side-channel.…”

Passive state preparation in the Gaussian-modulated coherent-states quantum key distribution

“…It seems reasonable to believe that a thermal source operated in CW mode will show better stability than optical modulators operated at high speed. How to deal with the source imperfections in CV-QKD has drawn much attention [36,38,[47][48][49][50]. We leave this as a future research topic.…”

Feasibility of quantum key distribution with macroscopically bright coherent light

“…The two scenarios can in principle be combined so that part of the modes are modulated and a nonequivalent part of the modes is matched to a generally multimode LO. However, if the same modulation is applied to the signal modes and some of the modes do not arrive at the detection, this may lead to side channels concerned with excessive modulation in CV QKD [32] and should be avoided. In our work we therefore study the cases when either only matched modes or all the modes are modulated and the modulation is different in different modes so that the side channel is ruled out (independent multimode modulation and joint homodyne detection is discussed in context of CV QKD in [15]).…”