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Optics Letters

Compromising quantum clock synchronization by a slow and persistent asymmetric channel delay strategy

Hui Han, Haotian Teng, Hailong Xu, Jinquan Huang, Yuanmei Xie, Yichen Zhang, Baokang Zhao, Wanrong Yu, Bo Liu, Shuhui Chen

Published Jul 1, 2026• 0 citations
Publisher

Abstract

Quantum clock synchronization achieves picosecond-level precision by compensating for channel delays based on reciprocity, which is intrinsically vulnerable to asymmetric delay attacks. In this Letter, we propose a slow and persistent asymmetric channel delay strategy that superimposes delays onto the channel, thereby producing a subtle, time-varying clock-offset error that can remain concealed within intrinsic timing fluctuations. In a 50 km round-trip quantum clock synchronization experiment, we demonstrate that the proposed attack successfully evades both threshold alarms and sliding window anomaly detection under varying delay intensities and modulation periods. This work uncovers key vulnerabilities in synchronization protocols under customizable attacks and provides a foundation for developing secure and resilient quantum clock synchronization systems.

DOI
10.1364/ol.599332

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Quantum clock synchronization achieves picosecond-level precision by compensating for channel delays based on reciprocity, which is intrinsically vulnerable to asymmetric delay attacks. In this Letter, we propose a slow and persistent asymmetric channel delay strategy that superimposes delays onto the channel, thereby producing a subtle, time-varying clock-offset error that can remain concealed within intrinsic timing fluctuations. In a 50 km round-trip quantum clock synchronization experiment, we demonstrate that the proposed attack successfully evades both threshold alarms and sliding window anomaly detection under varying delay intensities and modulation periods. This work uncovers key vulnerabilities in synchronization protocols under customizable attacks and provides a foundation for developing secure and resilient quantum clock synchronization systems.
We demonstrate an attack on a clock synchronization protocol that attempts to detect tampering of the synchronization channel using polarization-entangled photon pairs. The protocol relies on a symmetrical channel, where propagation delays do not depend on the propagation direction, for correctly deducing the offset between clocks—a condition that could be manipulated using optical circulators, which rely on static magnetic fields to break the reciprocity of propagating electromagnetic fields. Despite the polarization transformation induced within a set of circulators, our attack creates an error in time synchronization while evading detection.
The round-trip fiber time synchronization system is preferred for high-precision time synchronization due to its easier implementation compared to bidirectional systems. However, its link-symmetry assumption renders it susceptible to asymmetric delay attacks, posing a serious threat to the system that current encryption methods cannot effectively prevent. This letter proposes a controllable asymmetric channel attack scheme for the round-trip fiber time synchronization system. The objective is to assess the impact of asymmetry on time synchronization accuracy by measuring the time difference between the local and remote ends of the system. Experimental results demonstrate that by employing asymmetric channels, the synchronization accuracy of the system can be reduced to 1.01 ns, 2.09 ns, and 5.06 ns, respectively, from the initial accuracy of 52.40 ps. This work highlights the critical security issue in round-trip systems, emphasizing the urgent need for effective countermeasures against asymmetric delay attacks.
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Abstract Establishing and maintaining a common time reference across spatially separated devices is a prerequisite for networked quantum experiments and secure communications. Classical two-way timing protocols such as network time protocol or precision time protocol are vulnerable to asymmetric channel delays and cannot provide the picosecond-level precision demanded by quantum repeater networks. We propose and numerically evaluate a quantum-enhanced clock synchronization protocol based on attenuated weak coherent pulses (WCPs) and bidirectional Hong–Ou–Mandel (HOM) interferometry. Our simulations assume telecom-band photons (1550 nm) with a temporal width of 10.0 ns, a repetition rate of f = 10 MHz, effective mean photon number µ = 1.0, detector efficiency η = 85 % , detector timing jitter of 150 ps and channel loss 0.2 dB km −1 . We simulate that sub-nanosecond clock-offset accuracy and precision can be achieved under these operating conditions. This work demonstrates that high-repetition-rate WCPs combined with HOM interference can provide flexible and secure quantum clock synchronization at sub-nanosecond precision.
The information theoretically secure Kirchhoff-law-Johnson-noise (KLJN) key exchange scheme, similarly to quantum key distribution (QKD), is also potentially vulnerable against clock attacks, where Eve takes over the control of clock synchronization in the channel. This short note aims to introduce a time synchronization protocol scheme for Alice and Bob, which is resistant against arbitrary time delay attacks, both symmetric and asymmetric ones. We propose and explore various ways of clock synchronization for the KLJN system and propose an ultimate protocol that preserves time and hardware integrity under arbitrary attacks.
High-precision clock synchronization is essential for a wide range of network-distributed applications. In the quantum space, these applications include communication, sensing, and positioning. However, current synchronization techniques are vulnerable to attacks, such as intercept-resend attacks, spoofing, and delay attacks. Here, we propose and experimentally demonstrate a new quantum secure time transfer (QSTT) system, subsequently used for clock synchronization, that largely negates such attacks. Novel to our system is the optimal use of self-generated quantum keys within the QSTT to information-theoretically secure the maximum amount of timing data; as well as the introduction, within a hybrid quantum/post-quantum architecture, of an information-theoretic secure obfuscated encryption sequence of the remaining timing data. With these enhancements, we argue that our new system represents the most robust implementation of QSTT to date.