Composable security proof for continuous-variable quantum key distribution with coherent states

TL;DR

This paper provides the first composable security proof for continuous-variable quantum key distribution with coherent states, demonstrating security against general attacks and confirming Gaussian attacks' optimality asymptotically.

Contribution

It introduces a novel composable security proof for CV-QKD with coherent states, applicable to finite-size settings and general attacks.

Findings

Secret key rate converges to the Holevo bound in large blocks

Security against general attacks established using de Finetti or Postselection techniques

Parameter estimation method is assumption-free and broadly applicable

Abstract

We give the first composable security proof for continuous-variable quantum key distribution with coherent states against collective attacks. Crucially, in the limit of large blocks the secret key rate converges to the usual value computed from the Holevo bound. Combining our proof with either the de Finetti theorem or the Postselection technique then shows the security of the protocol against general attacks, thereby confirming the long-standing conjecture that Gaussian attacks are optimal asymptotically in the composable security framework. We expect that our parameter estimation procedure, which does not rely on any assumption, will find applications elsewhere, for instance for the reliable quantification of continuous-variable entanglement in finite-size settings.