Geometric phase-assisted simple phase compensation enabling quantum key distribution using phase-shifted Bell states

TL;DR

This paper introduces a simple geometric phase-based method for phase compensation in entanglement-based QKD, significantly reducing QBER and enhancing secure key rates in practical quantum communication systems.

Contribution

The authors propose a versatile phase-compensation scheme using geometric phase control that effectively eliminates relative phases in Bell states, improving entanglement fidelity and QKD security.

Findings

Achieved Bell state fidelity exceeding 95% in experiments.

Reduced QBER below 11%, meeting security thresholds for QKD.

Demonstrated robustness and practicality of the phase compensation method.

Abstract

Entanglement-based quantum key distribution (QKD) relies on the distribution of high-fidelity maximally entangled Bell states, typically generated via spontaneous parametric down-conversion (SPDC). In practical systems, unwanted relative phases arise from birefringence, pump-beam contributions, imperfect photon-pair generation, transmission through physical channels, and collection, transforming Bell states into phase-shifted states. This degrades interference visibility, increases the quantum bit error rate (QBER), and limits secure key generation. Conventional compensation techniques, such as birefringent crystals, interferometric stabilization, and spatial light modulators, are often impractical in real-world deployments. Here, we demonstrate a simple and versatile phase-compensation scheme that can be implemented at either the source or the receiver to eliminate arbitrary relative phases in Bell states. We theoretically and experimentally quantify the dependence of QBER in the BBM92 protocol on the relative phase and show that geometric-phase-based control can effectively restore entanglement quality. In a proof-of-concept experiment using a nondegenerate polarization Bell state, we achieve a fidelity exceeding 95% and reduce QBER below the 11% security threshold required for secure QKD. This robust approach enables practical phase control in entangled-photon systems and can be extended to time-bin QKD via time-polarization mapping, offering a promising route toward stable, low-QBER quantum communication.