Simple and secure quantum key distribution with biphotons

TL;DR

This paper explores secure quantum key distribution using biphotons to encode qutrits, demonstrating protocols that are nearly as effective as ideal systems and providing the first rigorous security analysis against general attacks.

Contribution

It introduces practical biphoton-based qutrit QKD protocols and proves their security against coherent attacks, expanding the error tolerance beyond previous qubit limits.

Findings

Security proven for a protocol with 17.7% error rate

Protocols can be integrated into existing single photon systems

First rigorous security analysis of qutrit QKD against general attacks

Abstract

The best qubit one-way quantum key distribution (QKD) protocol can tolerate up to 14.1% in the error rate. It has been shown how this rate can be increased by using larger quantum systems. The polarization state of a biphoton can encode a three level quantum system - a qutrit. The realization of a QKD system with biphotons encounters several problems in generating, manipulating and detecting such photon states. We define those limitations and find within them a few protocols that perform almost as well as the ideal qutrit protocol. One advantage is that these protocols can be implemented with minor modifications into existing single photon systems. The security of one protocol is proved for the most general coherent attacks and the largest acceptable error rate for this protocol is found to be around 17.7%. This is the first time, to the best of our knowledge, that the security of qutrit QKD protocols is rigorously analyzed against general attacks.