# Practical quantum key distribution with non-phase-randomized coherent states

## TL;DR

This paper introduces a six-coherent-state phase-encoding QKD protocol that can tolerate high losses, making coherent-state QKD more viable for long-distance secure communication with realistic system parameters.

## Contribution

It proposes a new six-coherent-state protocol with enhanced loss tolerance and a security proof based on semi-definite programming, extending the feasible distance for coherent-state QKD.

## Key findings

- Tolerates up to 38 dB loss with realistic parameters
- Can handle up to 56 dB loss with zero noise
- Suggests coherent-state QKD as a promising high-speed secure method

## Abstract

Quantum key distribution (QKD) based on coherent states is well known for its implementation simplicity, but it suffers from loss-dependent attacks based on optimal unambiguous state discrimination. Crucially, previous research has suggested that coherent-state QKD is limited to short distances, typically below 100 km assuming standard optical fiber loss and system parameters. In this work, we propose a six-coherent-state phase-encoding QKD protocol that is able to tolerate the total loss of up to 38 dB assuming realistic system parameters, and up to 56 dB loss assuming zero noise. The security of the protocol is calculated using a recently developed security proof technique based on semi-definite programming, which assumes only the inner-product information of the encoded coherent states, the expected statistics, and that the measurement is basis-independent. Our results thus suggest that coherent-state QKD could be a promising candidate for high-speed provably-secure QKD.