Practical security analysis of two-way quantum key distribution protocols based on non-orthogonal states

TL;DR

This paper analyzes the security of two-way quantum key distribution protocols using non-orthogonal states, demonstrating that increasing bases does not enhance security and introducing a new six-state protocol with security bounds.

Contribution

It provides a new, simplified security proof for a recent TWQKD protocol and introduces a novel six-state protocol with analytical security bounds.

Findings

Security of LM05 protocol is not improved by more than two bases.

A new proof of unconditional security for a recent TWQKD protocol.

Introduction of the TWQKD six-state protocol with security bounds.

Abstract

Within the broad research scenario of quantum secure communication, Two-Way Quantum Key Distribution (TWQKD) is a relatively new proposal for sharing secret keys that is not fully explored yet. We analyse the security of TWQKD schemes that use qubits prepared in non-orthogonal states to transmit the key. Investigating protocols that employ an arbitrary number of bases for the channel preparation, we show, in particular, that the security of the LM05 protocol can not be improved by the use of more than two preparation bases. We also provide a new proof of unconditional security for a deterministic TWQKD protocol recently proposed [Phys. Rev. A 88, 062302 (2013)]. In addition, we introduce a novel deterministic protocol named TWQKD six-state and compute an analytical lower bound (which can be tightened) for the maximum amount of information that an eavesdropper could extract in this case. An interesting advantage of our approach to the security analysis of TWQKD is the great simplicity and transparency of the derivations.