Intensity correlations in decoy-state BB84 quantum key distribution systems

TL;DR

This paper investigates how intensity correlations in decoy-state BB84 QKD systems, caused by increased repetition rates, leak information and significantly reduce the key rate, with higher-order correlations having a greater impact than previously thought.

Contribution

The study experimentally characterizes intensity correlations in industrial decoy-state BB84 QKD systems and demonstrates their substantial effect on key rate reduction, challenging prior assumptions.

Findings

Intensity correlations significantly reduce the asymptotic key rate.

Higher-order correlations impact the intensity more than nearest-neighbour correlations.

Experimental confirmation of the strong influence of higher-order correlations.

Abstract

The decoy-state method is a prominent approach to enhance the performance of quantum key distribution (QKD) systems that operate with weak coherent laser sources. Due to the limited transmissivity of single photons in optical fiber, current experimental decoy-state QKD setups increase their secret key rate by raising the repetition rate of the transmitter. However, this usually leads to correlations between subsequent optical pulses. This phenomenon leaks information about the encoding settings, including the intensities of the generated signals, which invalidates a basic premise of decoy-state QKD. Here we characterize intensity correlations between the emitted optical pulses in two industrial prototypes of decoy-state BB84 QKD systems and show that they significantly reduce the asymptotic key rate. In contrast to what has been conjectured, we experimentally confirm that the impact of higher-order correlations on the intensity of the generated signals can be much higher than that of nearest-neighbour correlations.