# Chau-Wang-Wong17 Scheme Is Experimentally More Feasible Than The Six-State Scheme

## TL;DR

This paper demonstrates that the four-dimensional qudit-based QKD scheme by Chau et al. is more experimentally feasible and tolerant to errors than the six-state scheme, supported by theoretical analysis and a proof-of-principle experiment.

## Contribution

It provides a detailed comparison showing the Chau et al. scheme's higher error tolerance and experimental advantages over the six-state scheme, including a practical implementation.

## Key findings

- Chau et al.'s scheme tolerates up to 21.6% four-dimensional dit error rate.
- The scheme has comparable secret key rates to the six-state scheme under ideal conditions.
- Experimental implementation shows practical advantages over traditional six-state QKD.

## Abstract

Recently, Chau et al. [Phys. Rev. A 95, 022311 (2017)] reported a quantum-key-distribution (QKD) scheme using four-dimensional qudits. Surprisingly, as a function of the bit error rate of the raw key, the secret key rate of this scheme is equal to that of the (qubit-based) six-state scheme under one-way classical communication using ideal apparatus in the limit of arbitrarily long raw key length. Here we explain why this is the case in spite of the fact that these two schemes are not linearly related to each other. More importantly, we find that in terms of the four-dimensional dit error rate of the raw key, the Chau et al.'s scheme can tolerate up to 21.6% using one-way classical communications, which is better than the Sheridan and Scarani's scheme [Phys. Rev. A 82, 030301(R) (2010)]. In addition, we argue the experimental advantages of the Chau et al. implementation over the standard six-state scheme and report a corresponding proof-of-principle experiment using passive basis selection with decoy states. We also compare our experiment with the recent high secret key rate implementation of the Sheridan and Scarani's scheme by Islam et al. [Sci. Adv. \text{3}, e1701491].