Trusted Noise in Continuous-Variable Quantum Key Distribution: a Threat and a Defense

We address the role of the phase-insensitive trusted preparation and detection noise in the security of a continuous-variable quantum key distribution, considering the Gaussian protocols on the basis of coherent and squeezed states and studying them in the conditions of Gaussian lossy and noisy channels. The influence of such a noise on the security of Gaussian quantum cryptography can be crucial, even despite the fact that a noise is trusted, due to a strongly nonlinear behavior of the quantum entropies involved in the security analysis. We recapitulate the known effect of the preparation noise in both direct and reverse-reconciliation protocols, as well as the detection noise in the reverse-reconciliation scenario. As a new result, we show the negative role of the trusted detection noise in the direct-reconciliation scheme. We also describe the role of the trusted preparation or detection noise added at the reference side of the protocols in improving the robustness of the protocols to the channel noise, confirming the positive effect for the coherent-state reverse-reconciliation protocol. Finally, we address the combined effect of trusted noise added both in the source and the detector.Comment: 25 pages, 9 figure

General treatment of Gaussian trusted noise in continuous variable quantum key distribution

Continuous Variable (CV) quantum key distribution (QKD) is a promising candidate for practical implementations due to its compatibility with the existing communication technology. A trusted device scenario assuming that an adversary has no access to imperfections such as electronic noises in the detector is expected to provide significant improvement in the key rate, but such an endeavor so far was made separately for specific protocols and for specific proof techniques. Here, we develop a simple and general treatment that can incorporate the effects of Gaussian trusted noises for any protocol that uses homodyne/heterodyne measurements. In our method, a rescaling of the outcome of a noisy homodyne/heterodyne detector renders it equivalent to the outcome of a noiseless detector with a tiny additional loss, thanks to a noise-loss equivalence well-known in quantum optics. Since this method is independent of protocols and security proofs, it is applicable to Gaussian-modulation and discrete-modulation protocols, to the finite-size regime, and to any proof techniques developed so far and yet to be discovered as well.Comment: 7 pages, 4 figure

Receiver Calibration and Quantum Random Number Generation for Continuous-variable Quantum Key Distribution

The desire for secure communications and the advent of quantum computing has spurred innovation into key-distribution technologies that are secure against future quantum computers. Computationally secure solutions based on post quantum algorithms and physically-secure solutions using either discrete-variable or continuous-variable quantum key distribution (CV-QKD) have been proposed. The attraction with CV-QKD systems in particular is the potential to leverage the vast knowledge base and access scaling benefits of photonic integration for conventional coherent optical communication for key distribution. CV-QKD requires detailed characterization of coherent receiver hardware, specifically noise generated by electronics and shot noise caused by the local oscillator (LO) laser. This work investigates the temporal stability of the receiver noise power which defines the amount of trusted noise in the quantum link used to compute the secret key rate (SKR). Depending on the noise power’s stability, this characterization must be repeated often, typically in the order of seconds. Therefore, this work explores the possibility of using the shot noise measurement as a source of quantum random numbers, which is required by a CV-QKD transceiver. This work enables further integration of the CV-QKD hardware, removing the need for a separate quantum random number generator (QRNG)

Modulation leakage vulnerability in continuous-variable quantum key distribution

Flaws in the process of modulation, or encoding of key bits in the quadratures of the electromagnetic light field, can make continuous-variable quantum key distribution systems susceptible to leakage of secret information. Here, we report such a modulation leakage vulnerability in a system that uses an optical in-phase and quadrature modulator to implement a single sideband encoding scheme. The leakage arises from the limited suppression of a quantum-information-carrying sideband during modulation. Based on the results from a proof-of-concept experiment, we theoretically analyse the impact of this vulnerability. Our results indicate that the leakage reduces the range over which a positive secret key can be obtained, and can even lead to a security breach if not properly taken into account. We also study the effectiveness of additional trusted noise as a countermeasure to this vulnerability.Comment: 12 pages, 6 figure