# Optimized protocol for twin-field quantum key distribution

## TL;DR

This paper presents a unified framework for twin-field quantum key distribution protocols by discretizing global phases, showing that increasing phase discretization extends communication distance but reduces short-distance key rates.

## Contribution

It introduces a unified view of different TF-QKD protocols through phase discretization, optimizing the trade-off between distance and key rate.

## Key findings

- Discretizing global phases unifies various TF-QKD protocols.
- Increasing phase discretization extends maximum communication distance.
- Higher phase discretization reduces secret key rate at short distances.

## Abstract

Twin-field quantum key distribution (TF-QKD) and its variant protocols are highly attractive due to the advantage of overcoming the rate-loss limit for secret key rates of point-to-point QKD protocols. For variations of TF-QKD, the key point to ensure security is switching randomly between a code mode and a test mode. Among all TF-QKD protocols, their code modes are very different, e.g. modulating continuous phases, modulating only two opposite phases, and sending or not sending signal pulses. Here we show that, by discretizing the number of global phases in the code mode, we can give a unified view on the first two types of TF-QKD protocols, and demonstrate that increasing the number of discrete phases extends the achievable distance, and as a trade-off, lowers the secret key rate at short distances due to the phase post-selection.