Rate-loss analysis of an efficient quantum repeater architecture

TL;DR

This paper provides a detailed analysis of a quantum repeater architecture for quantum key distribution, deriving exact secret-key rate expressions, demonstrating surpassing fundamental bounds, and analyzing the effects of device imperfections.

Contribution

It offers the first rigorous proof that a specific quantum repeater protocol exceeds the TGW bound and provides analytical tools for evaluating long-distance entanglement in quantum networks.

Findings

Achieves secret key rate surpassing the TGW bound.

Provides explicit analytical expressions for error propagation.

Extends analysis to sources with non-zero two-pair-emission probability.

Abstract

We analyze an entanglement-based quantum key distribution (QKD) architecture that uses a linear chain of quantum repeaters employing photon-pair sources, spectral-multiplexing, linear-optic Bell-state measurements, multi-mode quantum memories and classical-only error correction. Assuming perfect sources, we find an exact expression for the secret-key rate, and an analytical description of how errors propagate through the repeater chain, as a function of various loss and noise parameters of the devices. We show via an explicit analytical calculation, which separately addresses the effects of the principle non-idealities, that this scheme achieves a secret key rate that surpasses the TGW bound---a recently-found fundamental limit to the rate-vs.-loss scaling achievable by any QKD protocol over a direct optical link---thereby providing one of the first rigorous proofs of the efficacy of a repeater protocol. We explicitly calculate the end-to-end shared noisy quantum state generated by the repeater chain, which could be useful for analyzing the performance of other non-QKD quantum protocols that require establishing long-distance entanglement. We evaluate that shared state's fidelity and the achievable entanglement distillation rate, as a function of the number of repeater nodes, total range, and various loss and noise parameters of the system. We extend our theoretical analysis to encompass sources with non-zero two-pair-emission probability, using an efficient exact numerical evaluation of the quantum state propagation and measurements. We expect our results to spur formal rate-loss analysis of other repeater protocols, and also to provide useful abstractions to seed analyses of quantum networks of complex topologies.