Numerical security framework for quantum key distribution with bypass channels

TL;DR

This paper develops a numerical framework to evaluate quantum key distribution security when some signals bypass eavesdroppers, revealing new regimes where security can be improved under specific detector and source conditions.

Contribution

It introduces a general method for computing key rates in bypass scenarios, extending prior models and identifying conditions that enhance security in quantum key distribution.

Findings

BB84 performance does not improve under bypass constraints with single photons

Security improvements are found with detector efficiency mismatch

Weak coherent pulse implementations can benefit from bypass considerations

Abstract

Satellite based quantum key distribution (QKD) aims to establish secure key exchange over long distances despite significant technological challenges. To alleviate some of these challenges, Ghalaii et al. [PRX Quantum 4, 040320 (2023)] proposed that any airborne eavesdropper up to a certain size can be detected by classical monitoring techniques, limiting the transmission efficiencies of any undetected Eve. This creates a new QKD scenario in which some of the transmitted signal from Alice to Bob bypasses Eve entirely. In this manuscript, we develop a general framework for computing key rates in this "bypass" scenario for discrete variable protocols. We first numerically support a conjecture that the performance of BB84 with single photons does not improve under bypass constraints, and go on to find new regimes that do. Specifically, we find improvements when the receiver's detectors have an efficiency mismatch and when BB84 is implemented using weak coherent pulses under certain squashing assumptions. Technically, our framework is realized by including marginal constraints on the source to account for bypass effects, combined with existing numerical approaches for minimizing the key rate and squashing and dimension reduction techniques to handle photonic states of unbounded dimension.