Abstract
In recent years, there has been a great effort to prove the security of quantum key distribution (QKD) with a minimum number of assumptions. Besides its intrinsic theoretical interest, this would allow for larger tolerance against device imperfections in the actual implementations. However, even in this device-independent scenario, one assumption seems unavoidable, that is, the presence of a protected space devoid of any unwanted information leakage in which the legitimate parties can privately generate, process and store their classical data. In this paper we relax this unrealistic and hardly feasible assumption and introduce a general formalism to tackle the information leakage problem in most of existing QKD systems. More specifically, we prove the security of optical QKD systems using phase and intensity modulators in their transmitters, which leak the setting information in an arbitrary manner. We apply our security proof to cases of practical interest and show key rates similar to those obtained in a perfectly shielded environment. Our work constitutes a fundamental step forward in guaranteeing implementation security of quantum communication systems.
Highlights
It is well-known that two spatially separated users (Alice and Bob) can secretly communicate over a public channel if they own two identical random keys unknown to any third party
While the theoretical security of quantum key distribution (QKD) has been convincingly proven in recent years [5], in practice a QKD realisation cannot typically perfectly satisfy the requirements imposed by the theory
To illustrate how our formalism applies to real QKD systems, we investigate a particular form of information leakage, i.e., a Trojan-Horse attack (THA) that is feasible with current technology
Summary
It is well-known that two spatially separated users (Alice and Bob) can secretly communicate over a public channel if they own two identical random keys unknown to any third party. It is important to notice that the security of any form of QKD, including the two solutions above, relies on the assumption that Alice and Bob’s devices do not leak any unwanted information to the outside That is, their apparatuses must be inside private spaces that are well-shielded and inaccessible to Eve (see, e.g., [39]). There, security was proven under the assumption that this specific THA only affects the PM in the transmitter and leaves the other devices untouched This result cannot be exported to decoystate QKD and mdiQKD, where an additional method to modulate the intensity of the prepared signals is required. For a given model of PM and IM, one could readily use our technique to calculate the resulting secret key rate of the system This constitutes a fundamental step forward to guaranteeing the security of quantum cryptographic schemes using a PM, an IM or other analogous devices, in presence of information leakage. The paper contains Appendixes with calculations that are needed to derive the results in the main text
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