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International Conference on Transparent Optical Networks

Synchronization method for MDI-QKD: implementation and insights

Inés Meili Díaz García, Daniel Balado, Verónica Fernández, Daniel Cano Reol

Published Jul 6, 2025• 0 citations
Publisher

Abstract

Temporal synchronization in Quantum Key Distribution (QKD) remains a significant challenge, particularly in Measurement-Device-Independent QKD (MDI-QKD), where precise timing is crucial to generate the Bell states required for successful key exchange. Existing implementations employ various synchronization techniques, yet no standard approach has been universally adopted. To address this, we propose a real-time synchronization scheme that integrates atomic clocks with a time-fluctuation correcting algorithm. This algorithm post-processes events originated from the signals of Alice and Bob in Charlie, and applies a correction to compensate for temporal fluctuations, due to -mainly- frequency differrence between the atomic clocks, ensuring continuous and accurate synchronization throughout the (raw) key-sharing process. Our method enhances the efficiency of MDI-QKD by providing a robust, real-time synchronization mechanism.

DOI
10.1109/icton67126.2025.11125317

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Temporal synchronization in Quantum Key Distribution (QKD) remains a significant challenge, particularly in Measurement-Device-Independent QKD (MDI-QKD), where precise timing is crucial to generate the Bell states required for successful key exchange. Existing implementations employ various synchronization techniques, yet no standard approach has been universally adopted. To address this, we propose a real-time synchronization scheme that integrates atomic clocks with a time-fluctuation correcting algorithm. This algorithm post-processes events originated from the signals of Alice and Bob in Charlie, and applies a correction to compensate for temporal fluctuations, due to -mainly- frequency differrence between the atomic clocks, ensuring continuous and accurate synchronization throughout the (raw) key-sharing process. Our method enhances the efficiency of MDI-QKD by providing a robust, real-time synchronization mechanism.
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.
Measurement-device-independent quantum key distribution (MDI-QKD) is considered one of the most promising protocols due to its high performance and security. To implement the MDI-QKD system in practice, clock synchronization is a crucial step to correctly generate the secret keys. However, as practical applications advance, the complexity and cost of synchronization have become significant challenges for the implementation of MDI-QKD. In this study, we introduce a qubit-based synchronization algorithm specifically designed for MDI-QKD. By utilizing the property of Hong-Ou-Mandel interference, we achieve clock synchronization between parties of MDI-QKD without the need for additional hardware. The cost-effectiveness and simplicity of this approach are expected to significantly facilitate the practical deployment of MDI-QKD.
Measurement-device-independent quantum key distribution (MDI-QKD) eliminates detector side-channel attacks by relocating all measurements to an untrusted intermediate node. However, its practical implementation critically relies on picosecond-level temporal synchronization between spatially separated users. In this work, we present a physically motivated autonomous synchronization algorithm for fiber-based MDI-QKD networks that does not require auxiliary optical channels or shared clock references. The method exploits round-trip optical pulse propagation and statistical signal detection in the presence of Gaussian noise. We derive analytical expressions for false-alarm probabilities, quantify detection reliability, and demonstrate through numerical modeling that synchronization accuracy better than 10~ps is achievable for channel lengths up to 100~km with realistic optical power levels. The proposed approach improves the scalability and robustness of MDI-QKD architectures and is directly applicable to metropolitan and backbone quantum networks.
We propose a carrier synchronization scheme for continuous-variable measurement-device-independent quantum key distribution (CV-MDI-QKD) with a real local oscillator (LO). The middle node Charlie evaluates carrier frequency offsets and phase drift values based on the pilot signal sent by the legal parties Alice and Bob and a part of their public data. Alice and Bob then calibrate their modulation data according to the evaluation parameters to achieve carrier synchronization. We simulate the entire CV-MDI-QKD protocol and verify the feasibility of this scheme. The influences of pilot segment size and public data size on the excess noise are also analyzed. Besides, we experiment to measure the actual frequency offsets and phase drifts in the channel, which gives guidance on the actual implementation. This solution can ensure the implementation of CV-MDI-QKD with a real LO, paving the way for the practical application of this high-rate QKD protocol.
Measurement-device-independent quantum key distribution (MDI-QKD) eliminates detector side-channel attacks by relocating all measurements to an untrusted intermediate node. However, its practical implementation critically relies on picosecond-level temporal synchronization between spatially separated users. In this work, we present a physically motivated autonomous synchronization algorithm for fiber-based MDI-QKD networks that does not require auxiliary optical channels or shared clock references. The method exploits round-trip optical pulse propagation and statistical signal detection in the presence of Gaussian noise. We derive analytical expressions for false-alarm probabilities, quantify detection reliability, and demonstrate through numerical modeling that synchronization accuracy better than 10 ps is achievable for channel lengths up to 100 km with realistic optical power levels. The proposed approach improves the scalability and robustness of MDI-QKD architectures and is directly applicable to metropolitan and backbone quantum networks.
Measurement-device-independent quantum key distribution (MDI-QKD) eliminates detector side-channel attacks by relocating all measurements to an untrusted intermediate node. However, its practical implementation critically relies on picosecond-level temporal synchronization between spatially separated users. In this work, we present a physically motivated autonomous synchronization algorithm for fiber-based MDI-QKD networks that does not require auxiliary optical channels or shared clock references. The method exploits round-trip optical pulse propagation and statistical signal detection in the presence of Gaussian noise. We derive analytical expressions for false-alarm probabilities, quantify detection reliability, and demonstrate through numerical modeling that synchronization accuracy better than 10 ps is achievable for channel lengths up to 100 km with realistic optical power levels. The proposed approach improves the scalability and robustness of MDI-QKD architectures and is directly applicable to metropolitan and backbone quantum networks.