# Practical decoy-state method for twin-field quantum key distribution

## TL;DR

This paper presents a practical decoy-state method for twin-field quantum key distribution, demonstrating that using two to four decoy intensities suffices to outperform direct transmission limits with robustness against pulse fluctuations.

## Contribution

The authors derive analytical bounds for decoy-state parameters in TF-QKD with 2-4 decoy intensities, optimizing performance and robustness.

## Key findings

- Two decoy intensities are sufficient to surpass point-to-point capacity.
- Four decoys nearly match the ideal infinite-decoy scenario.
- Protocol is robust against intensity fluctuations.

## Abstract

Twin-Field (TF) quantum key distribution (QKD) represents a novel QKD approach whose principal merit is to beat the point-to-point private capacity of a lossy quantum channel, thanks to performing single-photon interference in an untrusted node. Indeed, recent security proofs of various TF-QKD type protocols have confirmed that the secret key rate of these schemes scales essentially as the square root of the transmittance of the channel. Here, we focus on the TF-QKD protocol introduced by Curty et al, whose secret key rate is nearly an order of magnitude higher than previous solutions. Its security relies on the estimation of the detection probabilities associated to various photon-number states through the decoy-state method. We derive analytical bounds on these quantities assuming that each party uses either two, three or four decoy intensity settings, and we investigate the protocol's performance in this scenario. Our simulations show that two decoy intensity settings are enough to beat the point-to-point private capacity of the channel, and that the use of four decoys is already basically optimal, in the sense that it almost reproduces the ideal scenario of infinite decoys. We also observe that the protocol seems to be quite robust against intensity fluctuations of the optical pulses prepared by the parties.