Security of decoy-state quantum key distribution with correlated intensity fluctuations

TL;DR

This paper improves the security analysis of decoy-state quantum key distribution systems affected by correlated intensity fluctuations, significantly increasing the achievable communication distance and key rate through a refined security proof and parameter estimation.

Contribution

It introduces a combined security proof and decoy-state analysis that better accounts for intensity correlations, enhancing performance over previous methods.

Findings

Key rate can be nearly as high as ideal when intensity fluctuation distribution is known.

Achievable distance is doubled compared to previous analyses in certain regimes.

Enhanced security proof yields more accurate parameter estimation.

Abstract

One of the most prominent techniques to enhance the performance of practical quantum key distribution (QKD) systems with laser sources is the decoy-state method. Current decoy-state QKD setups operate at GHz repetition rates, a regime where memory effects in the modulators and electronics that control them create correlations between the intensities of the emitted pulses. This translates into information leakage about the selected intensities, which cripples a crucial premise of the decoy-state method, thus invalidating the use of standard security analyses. To overcome this problem, a novel security proof that exploits the Cauchy-Schwarz constraint has been introduced recently. Its main drawback is, however, that the achievable key rate is significantly lower than that of the ideal scenario without intensity correlations. Here, we improve this security proof technique by combining it with a fine-grained decoy-state analysis, which can deliver a tight estimation of the relevant parameters that determine the secret key rate. This results in a notable performance enhancement, being now the attainable distance double than that of previous analyses for certain parameter regimes. Also, we show that when the probability density function of the intensity fluctuations, conditioned on the current and previous intensity choices, is known, our approach provides a key rate very similar to the ideal scenario, which highlights the importance of an accurate experimental characterization of the correlations.