Security of two-state and four-state practical quantum bit-commitment protocols

TL;DR

This paper analyzes cheating strategies against practical two-state and four-state quantum bit-commitment protocols under noisy channels, introducing new methods and comparing their security and resource efficiency.

Contribution

It introduces a novel cheating strategy based on post-measurement processes and compares the security of two-state and four-state protocols under realistic conditions.

Findings

Four-state protocol is more resource-efficient than two-state.

New cheating strategy outperforms direct state inference.

Protocol security is affected by source imperfections and channel noise.

Abstract

We study cheating strategies against a practical four-state quantum bit-commitment protocol and its two-state variant when the underlying quantum channels are noisy and the cheating party is constrained to using single-qubit measurements only. We show that simply inferring the transmitted photons' states by using the Breidbart basis, optimal for ambiguous (minimum-error) state discrimination, does not directly produce an optimal cheating strategy for this bit-commitment protocol. We introduce a new strategy, based on certain post-measurement processes, and show it to have better chances at cheating than the direct approach. We also study to what extent sending forged geographical coordinates helps a dishonest party in breaking the binding security requirement. Finally, we investigate the impact of imperfect single-photon sources in the protocols. Our study shows that, in terms of the resources used, the four-state protocol is advantageous over the two-state version. The analysis performed can be straightforwardly generalised to any finite-qubit measurement, with the same qualitative results.