# Asymptotic security of discrete-modulation protocols for continuous-variable quantum key distribution

## TL;DR

This paper proves the asymptotic security of discrete-modulation CV-QKD protocols with finite coherent states, providing formulas for secret-key rates and showing their rates approach Gaussian modulation as the constellation size increases.

## Contribution

It introduces a security proof for discrete-modulation CV-QKD with arbitrary finite constellations, extending previous results limited to two or three states.

## Key findings

- Secret-key rates approach Gaussian modulation rates with larger constellations.
- Achievable key rates scale proportionally to channel transmissivity in high-loss regimes.
- The method applies to lossy thermal bosonic channels.

## Abstract

We consider discrete-modulation protocols for continuous-variable quantum key distribution (CV-QKD) that employ a modulation constellation consisting of a finite number of coherent states and that use a homodyne or a heterodyne-detection receiver. We establish a security proof for collective attacks in the asymptotic regime, and we provide a formula for an achievable secret-key rate. Previous works established security proofs for discrete-modulation CV-QKD protocols that use two or three coherent states. The main constituents of our approach include approximating a complex, isotropic Gaussian probability distribution by a finite-size Gauss-Hermite constellation, applying entropic continuity bounds, and leveraging previous security proofs for Gaussian-modulation protocols. As an application of our method, we calculate secret-key rates achievable over a lossy thermal bosonic channel. We show that the rates for discrete-modulation protocols approach the rates achieved by a Gaussian-modulation protocol as the constellation size is increased. For pure-loss channels, our results indicate that in the high-loss regime and for sufficiently large constellation size, the achievable key rates scale optimally, i.e., proportional to the channel's transmissivity.