Finding and breaking the realistic rate-distance limit of continuous variable quantum key distribution

TL;DR

This paper investigates the rate-distance limit of continuous variable quantum key distribution, identifies key factors restricting it, and proposes methods to surpass these limits considering practical noise and finite-size effects.

Contribution

It introduces a realistic rate-distance limit considering excess noise and proposes a new secret key rate calculation method to break this limit.

Findings

Excess noise and calculation methods restrict the rate-distance limit.

A new method using pure excess noise improves the limit.

Finite-size analysis yields a tighter rate-distance limit.

Abstract

In this work, the rate-distance limit of continuous variable quantum key distribution is studied. We find that the excess noise generated on Bob's side and the method for calculating the excess noise restrict the rate-distance limit. Then, a realistic rate-distance limit is found. To break the realistic limit, a method for calculating the secret key rate using pure excess noise is proposed. The improvement in the rate-distance limit due to a higher reconciliation efficiency is analyzed. It is found that this improvement is dependent on the excess noise. From a finite-size analysis, the monotonicity of the Holevo bound versus the transmission efficiency is studied, and a tighter rate-distance limit is presented.