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

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.