Real-Field Hong-Ou-Mandel Interference of Indistinguishable Coherent Photons via Long Optical Injection-Locking over 50 km Fiber

TL;DR

This paper demonstrates a novel long optical injection-locking technique that achieves high-visibility Hong-Ou-Mandel interference between independent coherent photons over 50 km fiber, advancing secure quantum communication.

Contribution

The study introduces the long-OIL method for frequency locking of distant coherent photons, enabling near-ideal HOM interference in real-world fiber links for quantum key distribution.

Findings

Achieved 48% HOM interference visibility over 50 km fiber

Demonstrated frequency locking beyond the coherence length of the master laser

Showed potential for secure quantum communication without repeaters

Abstract

Measurement-device-independent quantum key distribution (MDI-QKD) has garnered significant attention for its potential to enable security-loophole-free quantum communication. Successful MDI-QKD protocols rely on performing a two-photon Bell-state measurement at an intermediate node, with a high-visibility Hong-Ou-Mandel (HOM) interference pattern between two independent coherent photons being crucial. In this study, we present a novel approach for developing indistinguishable coherent photon sources over 50 km of optical fiber in a real-world setting. We introduce the long optical injection-locking (long-OIL) technique, which enables frequency locking between two long-distance coherent photons beyond the coherence length of the master laser. Using the long-OIL technique, we achieved time-resolved HOM interference with a visibility of 48(2)%, approaching the theoretical 50% limit for two independent continuous-wave coherent photons. Our results demonstrate that the long-OIL platform is a promising solution for MDI-QKD with repeaterless secret key capacity.