Maximum tolerable excess noise in continuous-variable quantum key distribution and improved lower bound on two-way capacities

TL;DR

This paper establishes the maximum excess noise tolerable in continuous-variable quantum key distribution by deriving a new lower bound on two-way capacities of phase-insensitive bosonic Gaussian channels, enabling secure quantum communication without repeaters.

Contribution

It provides the first nonzero lower bound on two-way quantum capacity in regimes where reverse coherent information is negative, solving a key open problem in quantum communication.

Findings

Explicit lower bounds on two-way capacities for thermal channels.

Demonstrates entanglement distribution is possible beyond entanglement-breaking noise levels.

Develops an optimized protocol combining recurrence and hashing for entanglement distillation.

Abstract

The two-way capacities of quantum channels determine the ultimate entanglement and secret-key distribution rates achievable by two distant parties that are connected by a noisy transmission line, in absence of quantum repeaters. Since repeaters will likely be expensive to build and maintain, a central open problem of quantum communication is to understand what performances are achievable without them. In this paper, we find a new lower bound on the energy-constrained and unconstrained two-way quantum and secret-key capacities of all phase-insensitive bosonic Gaussian channels, namely thermal attenuator, thermal amplifier, and additive Gaussian noise, which are realistic models for the noise affecting optical fibres or free-space links. Ours is the first nonzero lower bound on the two-way quantum capacity in the parameter range where the (reverse) coherent information becomes negative, and it shows explicitly that entanglement distribution is always possible when the channel is not entanglement breaking. This completely solves a crucial open problem of the field, namely, establishing the maximum excess noise which is tolerable in continuous-variable quantum key distribution. In addition, our construction is fully explicit, i.e. we devise and optimise a concrete entanglement distribution and distillation protocol that works by combining recurrence and hashing protocols