Post-selective attack with multi-mode projection onto Fock subspace

TL;DR

This paper analyzes a post-selective attack on phase-encoded quantum key distribution protocols using multimode projection onto Fock subspace, revealing how an eavesdropper can probabilistically extract information based on key protocol parameters.

Contribution

It provides analytical expressions for the eavesdropper's accessible information and discusses potential countermeasures, advancing understanding of security vulnerabilities in quantum key distribution.

Findings

Eavesdropper's information depends on mean photon number, phase separation, and optical loss.

Analytical formulas for attack effectiveness are derived.

Countermeasures against the attack are proposed.

Abstract

In this work we present a comprehensive analysis of a post-selective attack on quantum key distribution protocols employing phase-encoded linearly independent coherent states (or similar alternatives). The attack relies on multimode projection onto a Fock subspace and enables probabilistic extraction of information by an eavesdropper. We derive analytical expressions for the information accessible to the adversary and show that it depends only on three protocol parameters: the mean photon number of the signal states, the phase separation in the information basis, and the expected optical loss of the quantum channel. Several optical realizations of phase-encoded quantum key distribution protocols are analyzed to illustrate the applicability of the results. Possible countermeasures against the proposed attack are also discussed.