Daniil Trefilov

Chip-based quantum key distribution (QKD) systems offer improved efficiency but may also introduce previously unrecognized security vulnerabilities. In this work, we identify and experimentally characterize cross-polarization-intensity (CPI) correlations in a real-world chip-based QKD system. Moreover, we introduce a security analysis that incorporates CPI correlations and apply it to evaluate the performance of an integrated high-speed QKD system. Our results emphasize the need for rigorous security assessments in chip-based QKD implementations.

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. Current experimental decoy-state QKD setups increase their secret key rate by raising the repetition rate of the transmitter, which can lead to correlations between subsequently emitted optical pulses. This phenomenon leaks information about the encoding settings, including the intensities of the generated signals, thus invalidating a basic premise of decoy-state QKD. Here, we experimentally characterize intensity correlations between the nearest-neigbouring optical pulses in two commercial prototypes of decoy-state BB84 QKD systems and show that they significantly reduce the asymptotic key rate. In addition, we study intensity correlations between pulses spaced further apart (higher-order correlations) and find that, in contrast to what has been conjectured, their impact on the intensity of the generated signals can be much higher than that of the nearest-neighbour (first-order) correlations.

We report recent advances in the development of certification for quantum key distribution (QKD) systems. We give an example of a commercial QKD system that we have analysed for possible loopholes, improved to close the vulnerabilities identified, and designed a set of tests for that can be used by a certification lab [arXiv:2310.20107]. We explain some of the testbenches in this lab, such as an ultrawide spectral characterisation testbench, automated detector testing, and laser damage testbench that verifies the quality of a power limiter. This work is in line with the requirements of the ISO standard for QKD and paves the way for the creation of certification services.