A security proof of continuous-variable QKD using three coherent states

TL;DR

This paper proposes a new ternary continuous-variable quantum key distribution protocol using three coherent states, providing a detailed security proof and comparing its performance to existing schemes.

Contribution

It extends binary coherent state QKD to a ternary scheme and offers a comprehensive security proof and performance analysis.

Findings

Lower bounds on secret key rate under collective attacks

Comparison with other QKD schemes and fundamental limits

Practical benefits of ternary encoding in CV-QKD

Abstract

We introduce a ternary quantum key distribution (QKD) protocol and asymptotic security proof based on three coherent states and homodyne detection. Previous work had considered the binary case of two coherent states and here we nontrivially extend this to three. Our motivation is to leverage the practical benefits of both discrete and continuous (Gaussian) encoding schemes creating a best-of-both-worlds approach; namely, the postprocessing of discrete encodings and the hardware benefits of continuous ones. We present a thorough and detailed security proof in the limit of infinite signal states which allows us to lower bound the secret key rate. We calculate this is in the context of collective eavesdropping attacks and reverse reconciliation postprocessing. Finally, we compare the ternary coherent state protocol to other well-known QKD schemes (and fundamental repeaterless limits) in terms of secret key rates and loss.