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arXiv:2502.04772 · Optics Express

Real-field Hong–Ou–Mandel interference of indistinguishable coherent photons via long optical injection-locking over 50 km fiber

Seoyeon Yang, Danbi Kim, Hansol Jeong, Han Seb Moon

Published Jun 16, 2025• 0 citations
arXivHTMLPDF

Abstract

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

DOI
10.1364/oe.559147

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Measurement-device-independent quantum key distribution (MDI-QKD) has garnered significant attention for enabling security-loophole-free quantum communication. Successful MDI-QKD protocols rely on performing a two-photon Bell-state measurement at an intermediate node, where achieving a high-visibility Hong-Ou-Mandel (HOM) interference pattern between two independent coherent photons is crucial. In this study, we present an approach for developing indistinguishable coherent photon sources over 50 km of optical fiber under real-world conditions. We introduce the long optical injection-locking (long-OIL) technique, which enables frequency locking between two long-distance coherent photons separated by distances exceeding the coherence length of the master laser. Utilizing the long-OIL technique, we achieved time-resolved HOM interference with a visibility of 48(2)%, which is close to the theoretical limits of 50% for two independent continuous-wave coherent photons. Our results indicate that the long-OIL platform is a promising solution for MDI-QKD with repeaterless secret-key capacity.
Due to the simplicity of Hong-Ou-Mandel (HOM) interferometer setup, it becomes very important in various quantum-based applications, especially when the photon indistinguishability is required such as measurement device independent quantum key distribution (MDI-QKD). In the present work, the HOM experiment setup is designed to study the quantum interference between two indistinguishable photons (frequency, time of arrival and polarization should be same for both photons) coming from two independent laser sources (attenuated laser). The aim is to achieve high visibility as it indicates the photon indistinguishability from two independent sources. Theoretically, the visibility is 50%50 \%50% for indistinguishable photons from attenuated laser sources but in the experiment, due to various devices imperfections and other factors it is less than 50%. The HOM visibility achieved from our setup is 40%40 \%40% tested for a distance of 20 km over standard single mode fiber (SMF). The main point to be noticed in this work is that, this experiment is performed using single photon avalanche diode.
Breakthroughs in chip integration technology have accelerated the practical deployment and scalable implementation of quantum key distribution (QKD). On-chip quantum light sources are critical components of fully integrated QKD systems. Measurement-Device-Independent QKD (MDI-QKD) offers significant advantages in network communication by eliminating vulnerabilities to side-channel attacks at the detector side, while relying on high-visibility Hong-Ou-Mandel interference between photons emitted by two independent lasers. Here, we present an innovative design of the silicon-based hybrid integrated self-injection locking (SIL) laser chip that achieves high-frequency-stability operation with the frequency detuning between independent lasers suppressed to within 15 MHz, enabling high-visibility HOM interference in MDI-QKD. Leveraging this design, we experimentally demonstrate a loss-tolerant MDI-QKD system employing the setting-choice-independent correlation security framework explicitly accounting for state-preparation uncertainty. The system achieves a finite-key secret rate of 17 b/s under 34 dB channel loss (or 170-km standard fiber), surpassing previously reported MDI-QKD experiments that incorporated state preparation uncertainty under equivalent transmission loss conditions. Our result makes a promising contribution to practical applications of fully integrated quantum communication networks.
Hong-Ou-Mandel (HOM) interference is one of the most important experimental phenomena in quantum optics. It has drawn considerable attention with respect to quantum cryptography and quantum communication because of the advent of the measurement device independent (MDI) quantum key distribution (QKD) protocol. Here, we realize HOM interference, having a visibility of approximately 38.1%, using two independent heralded single-photon sources (HSPSs). The HOM interference between two independent HSPSs is a core technology for realizing the long-distance MDI QKD protocol, the quantum coin-tossing protocol, and other quantum cryptography protocols.
Hong-Ou-Mandel interference is essential for various quantum communication protocols, including quantum teleportation, entanglement swapping, and measurement-device-independent (MDI) quantum key distribution (QKD). A significant challenge is producing single photons that remain indistinguishable over long distances. Time jitter introduced in synchronization between remote nodes reduces temporal overlap, making photons no longer indistinguishable. To mitigate this effect, previous solutions are forced to increase the temporal overlap, typically by using narrowband filtering strategies to extend photon coherence time. However, this dramatically decreases photon rates, making them impractical for real-life applications. Here we address this dilemma by employing common laser pulses to generate intrinsically synchronized single photons, eliminating the need for extended coherence time. Without compromising photon indistinguishability, we achieved a four-fold coincidence count rate more than two orders of magnitude higher than previous methods over 50 kilometers of fiber transmission. This breakthrough enabled the implementation of MDI-QKD with passive decoy states, demonstrating the viability of our approach.
Interference between two completely independent photons lies at the heart of many photonic quantum information applications such as quantum repeaters, teleportation, and quantum key distribution. Here, we report the observation of Hong–Ou–Mandel (HOM) interference with two independent continuous-wave coherent light sources that are neither synchronized nor share any common reference. To prepare indistinguishable photons from two independent laser sources, we employ high-precision frequency-stabilization techniques using the 5 S 1 / 2 ( F = 3 ) − 5 P 1 / 2 ( F ′ = 3 ) transition line of Rb 85 atoms. We successfully observe an HOM interference fringe with two independent continuous-wave coherent photons originating from either the frequency-locked or the frequency-modulated lasers. An interference fringe involving two-photon beating is also observed when the frequency difference between the two interfering photons is beyond the spectral bandwidth of the individual coherent photons. We carry out further experiments to verify the robustness of the source preparation regardless of the separation distance between the two independent photon sources.
Recently, the Hong–Ou–Mandel (HOM) interference between two independent weak coherent pulses (WCPs) has been paid much attention due to the measurement-device-independent (MDI) quantum key distribution (QKD). Using classical wave theory, articles reported before show that the visibility of this kind of HOM-type interference is ≤ 50%. In this work, we analyze this kind of interference using quantum optics, which reveals more details compared to the wave theory. Analyses confirm the maximum visibility of 50%. And we conclude that the maximum visibility of 50% comes from the two single-photon states in WCPs, without considering the noise. In the experiment, we successfully approach the visibility of 50% by using WCPs splitting from the single pico-second laser source and phase scanning. Since this kind of HOM interference is immune to slow phase fluctuations, both the realized and proposed experiment designs can provide stable ways of high-resolution optical distance detection.