# Parameter regimes for a single sequential quantum repeater

## TL;DR

This paper evaluates the performance of a single sequential quantum repeater for quantum key distribution, introducing benchmarks and methods to enhance key rates, thus guiding practical implementation of quantum repeaters.

## Contribution

It provides a detailed assessment of a natural quantum repeater setup, introducing benchmarks and methods like cut-offs and advantage distillation to improve key rates.

## Key findings

- Achievable key rates surpass benchmarks with proposed methods.
- Finite-energy, thermal noise, and loss considerations are critical for repeater performance.
- Guidelines for implementing effective quantum repeaters are established.

## Abstract

Quantum key distribution allows for the generation of a secret key between distant parties connected by a quantum channel such as optical fibre or free space. Unfortunately, the rate of generation of a secret key by direct transmission is fundamentally limited by the distance. This limit can be overcome by the implementation of so-called quantum repeaters. Here, we assess the performance of a specific but very natural setup called a single sequential repeater for quantum key distribution. We offer a fine-grained assessment of the repeater by introducing a series of benchmarks. The benchmarks, which should be surpassed to claim a working repeater, are based on finite-energy considerations, thermal noise and the losses in the setup. In order to boost the performance of the studied repeaters we introduce two methods. The first one corresponds to the concept of a cut-off, which reduces the effect of decoherence during storage of a quantum state by introducing a maximum storage time. Secondly, we supplement the standard classical post-processing with an advantage distillation procedure. Using these methods, we find realistic parameters for which it is possible to achieve rates greater than each of the benchmarks, guiding the way towards implementing quantum repeaters.