Composable security of two-way continuous-variable quantum key distribution without active symmetrization

TL;DR

This paper introduces a unified framework for continuous-variable quantum key distribution protocols, proving their security without active symmetrization, and demonstrates composable security for two-way protocols against general attacks.

Contribution

It provides a general, covariant framework for CV-QKD protocols and establishes their composable security without the need for active symmetrization procedures.

Findings

Protocols are covariant under $U(n)$, enabling Gaussian de Finetti reduction.

Security against general attacks is proven for two-way CV-QKD.

No active symmetrization is necessary, simplifying implementation.

Abstract

We present a general framework encompassing a number of continuous-variable quantum key distribution protocols, including standard one-way protocols, measurement-device-independent protocols as well as some two-way protocols, or any other continuous-variable protocol involving only a Gaussian modulation of coherent states and heterodyne detection. The main interest of this framework is that the corresponding protocols are all covariant with respect to the action of the unitary group $U(n)$, implying that their security can be established thanks to a Gaussian de Finetti reduction. In particular, we give a composable security proof of two-way continuous-variable quantum key distribution against general attacks. We also prove that no active symmetrization procedure is required for these protocols, which would otherwise make them prohibitively costly to implement.