Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

npj Quantum Information

Experimental demonstration of long distance quantum communication with independent heralded single photon sources

Xiao-Hai Zhan, Zhen-Qiu Zhong, Jianyu Ma, Shuang Wang, Zhen−Qiang Yin, Wei Chen, De‐Yong He, Guang‐Can Guo, Zheng‐Fu Han

Published May 8, 2025• 26 citations
Publisher

Abstract

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.

DOI
10.1038/s41534-025-01025-w

Related papers

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 +1 more

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.

arXiv:2502.04772•Optics Express•Jun 16, 2025•0 citations

Practical Decoy-State Memory-Assisted Measurement-Device-Independent Quantum Key Distribution

Ming-Shuo Sun, Chunhui Zhang, Hua-Jian Ding +3 more

The measurement-device-independent quantum key distribution (MDI QKD) can resist all attacks against measurement devices. However, its practical performance is still limited due to very low coincidence counting rates in Bell-state measurements. In this paper, we propose a practical memory-assisted MDI QKD protocol by cooperating the heralded single-photon sources and an all-optical quantum memory. By making use of the heralding characteristic of heralded single-photon sources, the earlier photon will be stored in the Sagnac optical quantum memory, and then released till another photon's arrival. Moreover, by combining with the decoy-state method, the present work exhibits significantly improved key rates and transmission distances compared with all existing MDI QKD protocols without using quantum memories. Therefore, this work can provide valuable references for practical implementation of long-distance quantum communications.

Physical Review Applied•Aug 11, 2023•9 citations

Benchmarking Single-Photon Sources for Measurement-Device-Independent Quantum Key Distribution

Janka Memmen, Yvonne Lasars, Tobias Heindel +2 more

Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) enables secure communication without trusting the measurement devices, but its performance critically depends on the quality of the employed photon sources. We compare MDI-QKD implementations based on Weak Coherent Pulses (WCPs) with those employing Quantum Dot (QD) single-photon sources. WCP-based schemes are fundamentally limited by Poissonian photon-number statistics, imposing a trade-off between single- and multi-photon emissions, whereas QD sources exhibit sub-Poissonian statistics with low antibunching parameter $g^{(2)}(0)$. Using a decoy-state analysis for both source types, we find that even at moderate mean photon numbers, QD-based implementations outperform the best-case WCP configuration - where the mean photon number is optimised at each transmission distance - across the full range of transmission distances. We further incorporate the effect of imperfect photon indistinguishability via Hong-Ou-Mandel interference visibilities into the security framework and demonstrate that both source types are highly sensitive to even small deviations from ideal indistinguishability, significantly affecting achievable key rates and maximum transmission distance.

quant-pharXiv:2610.42041•Nov 2, 2026

Long-Distance Free-Space Measurement-Device-Independent Quantum Key Distribution

Yuan Wen Cao, Yu-Huai Li, Kui-Xing Yang +20 more

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

arXiv:2006.05088•Physical Review Letters•Dec 23, 2020•173 citations

Highly Efficient Quantum Key Distribution Immune to All Detector Attacks

Wen-Fei Cao, Yi-Zheng Zhen, Zheng, Yu-Lin +4 more

Vulnerabilities and imperfections of single-photon detectors have been shown to compromise security for quantum key distribution (QKD). The measurement-device-independent QKD (MDI-QKD) appears to be the most appealing solution to solve the issues. However, in practice one faces severe obstacles of having significantly lower key generation rate, difficult two photon interferences, and remote synchronization etc. In this letter, we propose a highly efficient and simple quantum key distribution scheme to remove all of these drawbacks. Our proposal can be implemented with only small modifications over the standard decoy BB84 system. Remarkably it enjoys both the advantages of high key generation rate (being almost two orders of magnitude higher than that based on conventional MDI-QKD) comparable to the normal decoy system, and security against any detector side channel attacks. Most favorably one can achieve complete Bell state measurements with resort to single photon interference, which reduces significantly experimental costs. Our approach enables utilization of high speed and efficient secure communication, particularly in real-life scenario of both metropolitan and intercity QKD network, with an attack free fashion from arbitrary detector side channels.

arXiv:1410.2928•Oct 10, 2014•5 citations

Reduction of the two-photon temporal distinguishability for measurement-device-independent quantum key distribution

Haobo Ge, Akihisa Tomita, Atsushi Okamoto +1 more

Measurement-device-independent quantum key distribution (MDI-QKD) has been proven to protect legitimate users from attacks against measurement devices. The MDI-QKD requires that the two photons arriving at the instrument be indistinguishable. Precise time control is required to eliminate the distinguishability due to differences in photon arrival times. In the conventional methods, the time difference between photons is measured at a measuring instrument (Charlie), and a control signal is transmitted to the users (Alice and Bob). However, this method requires a long feedback loop, and the control may become unstable for long-distance transmission. This article proposes a method in which the photon arrival time difference is detected and controlled at Charlie. The reference signal for the time control is generated by an optical frequency comb in synchronization with the quantum signal. Therefore, the quantum signal photons can also be synchronized by synchronizing the reference signal pulses. A proof-of-principle experiment confirmed that the time synchronization accuracy required for protocol execution could be obtained. This proposal simplifies the implementation of the MDI-QKD.

Optics Letters•Jan 22, 2024•2 citations
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.
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.
The measurement-device-independent quantum key distribution (MDI QKD) can resist all attacks against measurement devices. However, its practical performance is still limited due to very low coincidence counting rates in Bell-state measurements. In this paper, we propose a practical memory-assisted MDI QKD protocol by cooperating the heralded single-photon sources and an all-optical quantum memory. By making use of the heralding characteristic of heralded single-photon sources, the earlier photon will be stored in the Sagnac optical quantum memory, and then released till another photon's arrival. Moreover, by combining with the decoy-state method, the present work exhibits significantly improved key rates and transmission distances compared with all existing MDI QKD protocols without using quantum memories. Therefore, this work can provide valuable references for practical implementation of long-distance quantum communications.
Measurement-Device-Independent Quantum Key Distribution (MDI-QKD) enables secure communication without trusting the measurement devices, but its performance critically depends on the quality of the employed photon sources. We compare MDI-QKD implementations based on Weak Coherent Pulses (WCPs) with those employing Quantum Dot (QD) single-photon sources. WCP-based schemes are fundamentally limited by Poissonian photon-number statistics, imposing a trade-off between single- and multi-photon emissions, whereas QD sources exhibit sub-Poissonian statistics with low antibunching parameter g(2)(0)g^{(2)}(0)g(2)(0). Using a decoy-state analysis for both source types, we find that even at moderate mean photon numbers, QD-based implementations outperform the best-case WCP configuration - where the mean photon number is optimised at each transmission distance - across the full range of transmission distances. We further incorporate the effect of imperfect photon indistinguishability via Hong-Ou-Mandel interference visibilities into the security framework and demonstrate that both source types are highly sensitive to even small deviations from ideal indistinguishability, significantly affecting achievable key rates and maximum transmission distance.
Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates, and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over a free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step toward satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.
Vulnerabilities and imperfections of single-photon detectors have been shown to compromise security for quantum key distribution (QKD). The measurement-device-independent QKD (MDI-QKD) appears to be the most appealing solution to solve the issues. However, in practice one faces severe obstacles of having significantly lower key generation rate, difficult two photon interferences, and remote synchronization etc. In this letter, we propose a highly efficient and simple quantum key distribution scheme to remove all of these drawbacks. Our proposal can be implemented with only small modifications over the standard decoy BB84 system. Remarkably it enjoys both the advantages of high key generation rate (being almost two orders of magnitude higher than that based on conventional MDI-QKD) comparable to the normal decoy system, and security against any detector side channel attacks. Most favorably one can achieve complete Bell state measurements with resort to single photon interference, which reduces significantly experimental costs. Our approach enables utilization of high speed and efficient secure communication, particularly in real-life scenario of both metropolitan and intercity QKD network, with an attack free fashion from arbitrary detector side channels.
Measurement-device-independent quantum key distribution (MDI-QKD) has been proven to protect legitimate users from attacks against measurement devices. The MDI-QKD requires that the two photons arriving at the instrument be indistinguishable. Precise time control is required to eliminate the distinguishability due to differences in photon arrival times. In the conventional methods, the time difference between photons is measured at a measuring instrument (Charlie), and a control signal is transmitted to the users (Alice and Bob). However, this method requires a long feedback loop, and the control may become unstable for long-distance transmission. This article proposes a method in which the photon arrival time difference is detected and controlled at Charlie. The reference signal for the time control is generated by an optical frequency comb in synchronization with the quantum signal. Therefore, the quantum signal photons can also be synchronized by synchronizing the reference signal pulses. A proof-of-principle experiment confirmed that the time synchronization accuracy required for protocol execution could be obtained. This proposal simplifies the implementation of the MDI-QKD.