Long-fiber Sagnac interferometers for twin-field quantum key distribution networks

Based on carriers that cannot be copied or eavesdropped without notice to the communicating parties, quantum key distribution (QKD) allows remote users to establish shared encryption keys with information-theoretic security. QKD networks are an important building block for large-scale quantum networks and have been studied extensively [5– 12 ]. However, their key rates are limited by the repeaterless bounds on the key rate scaling with channel loss.

Publication Date: March 19, 2025

Authors:

Reem Mandil, Li Qian, and Hoi-Kwong Lo

Abstract:

A Sagnac-loop structure can help overcome the major difficulty in the practical implementation of a twin-field quantum key distribution (TFQKD) network, namely, the need to stabilize the phase of a quantum state over many kilometers of fiber. Unfortunately, Rayleigh backscattering noise limits the signal-to-noise ratio for Sagnac systems containing long fibers and lossy photonic devices. Here, we solve this problem by sending optical pulses in long on-off bursts and using time postselection on measurements taken with free-run single-photon avalanche detectors. We also investigate the impact of the residual phase noise uncompensated by the Sagnac structure and find that the variance of the phase noise scales as loop length to the third power, verifying an existing calculation in the literature. We measure the interference visibility in Sagnac loops of varying length without active phase stabilization and achieve >97% visibility in 200-km ultralow-loss fiber, which is, to our knowledge, the longest fiber Sagnac interferometer demonstrated. Our results suggest that a Sagnac system is feasible for long-distance TFQKD networks, a key step towards the practical implementation of metropolitan quantum networks.

Related links:

Read this publication on the Physical Review Applied website

Return to CQIQC member publications.