Christopher Spiess, Sebastian Töpfer, Sakshi Sharma, Andrej Kržič, Meritxell Cabrejo‐Ponce, Uday Chandrashekara, Nico Döll, Daniel Rieländer, Fabian Steinlechner
Event synchronization is a ubiquitous task, with applications ranging from 5G technology to industrial automation and smart power grids. The emergence of quantum communication networks will further increase the demand for precise synchronization in the optical and electronic domains, which implies significant resource overhead, such as the requirement for ultrastable clocks or additional synchronization lasers. Here, we show how temporal correlations of photons may be harnessed for synchronization in quantum networks. We achieve stable synchronization jitter $<68$ ps with as few as 44 correlated detection events per 100-ms data package and demonstrate feasibility in realistic emulated high-loss link scenarios, including atmospheric turbulence. In contrast to previous work, this is accomplished without any external timing reference and only simple crystal oscillators. Our approach replaces the optical and electronic transmission of timing signals with classical communication and computer-aided postprocessing. It can be easily integrated into a wide range of quantum communication networks and could pave the way to future applications in entanglement-based secure time transmission.
Event synchronization is a ubiquitous task, with applications ranging from 5G technology to industrial automation and smart power grids. The emergence of quantum communication networks will further increase the demand for precise synchronization in the optical and electronic domains, which implies significant resource overhead, such as the requirement for ultrastable clocks or additional synchronization lasers. Here, we show how temporal correlations of photons may be harnessed for synchronization in quantum networks. We achieve stable synchronization jitter <68 ps with as few as 44 correlated detection events per 100-ms data package and demonstrate feasibility in realistic emulated high-loss link scenarios, including atmospheric turbulence. In contrast to previous work, this is accomplished without any external timing reference and only simple crystal oscillators. Our approach replaces the optical and electronic transmission of timing signals with classical communication and computer-aided postprocessing. It can be easily integrated into a wide range of quantum communication networks and could pave the way to future applications in entanglement-based secure time transmission.
Abstract Photonic quantum technology requires precise, time-resolved identification of photodetection events. In distributed quantum networks with spatially separated and drifting time references, achieving high precision is particularly challenging. Here we build on recent advances of using single-photons for time transfer and employ and quantify a fast postprocessing scheme designed to pulsed single-photon sources. We achieve an average root mean square synchronization jitter of 3.0 ps. The stability is comparable to systems with Rb vapor cell clocks with 19 ps at 1 s integration time, in terms of Allan time deviation. Remarkably, our stability is even better than classical high-precision time transfer, like the White Rabbit protocol, although we use significantly less signal (single-photon level). Our algorithms allow local processing of the data and do not affect the secure key rate. It compensates substantial clock imperfections from crystal oscillators and we foresee great potential for low signal scenarios. The findings are naturally suited to quantum communication networks and provide simultaneous time transfer without adding hardware or modifying the single-photon sources.
Quantum communication networks exploit single photons to generate secure encryption keys and can have very tight timing requirements [1]. These synchronization requirements are met through highly stable references, but there is ongoing research on using only single photons to enable synchronization [2]–[4]. The single photons come naturally with the quantum communication protocol and motivate its use for both data and time transfer. As point-to-point quantum communication scenarios rapidly expand to large networks, there is great demand for robust and resource-saving synchronization methods. In this contribution, we show that single photons can be indeed a solid carrier of timing information. We complement other publications [2], [5] by describing the performance of the synchronization approach and evaluating it on a 1.7 km free space and a 70 km deployed intercity fiber link. We demonstrate time deviations of 100 ps (1.7 km free-space link) and 174 ps (70 km fiber-link) at integration times of 1 second with only crystal oscillators, which is slightly larger than 54 ps in a lab environment (Fig. 1(a)). Our results provide advances towards a strong, independent, and secure quantum communication network without external hardware and a reliable operation under versatile link scenarios.
Time synchronization is a fundamental requirement in entanglement-based quantum networks, where the indistinguishability of photons in the time domain is essential for enabling Hong-Ou-Mandel interference and entanglement swapping. In addition to precise temporal alignment, it is equally crucial to ensure the stability of the reference clock over time, as even small fluctuations can degrade the overall performance of the network. In this work, we investigate the stability of clock synchronization for entanglement distribution based on entangled-photon sources operating in the telecommunication C-band. Temporal correlations between photon detection events are analyzed using time-tagged coincidence measurements, enabling the extraction of synchronization peaks and their long-term stability. Experimental results demonstrate that, once locked, the sources exhibit stable temporal correlations over an 8-hour acquisition period, with a maximum observed drift of approximately 120 ps, primarily associated with long fiber links. The width of the correlation peak remains consistent with detector jitter, indicating negligible additional system-induced temporal broadening. A central result of this work is the experimental synchronization between two entanglement-photon sources in a realistic metropolitan deployment, where entanglement distribution is performed over existing telecommunication infrastructure characterized by non-negligible losses and background noise. In this scenario, despite the presence of significant imperfections introduced by the metropolitan-scale fiber network, the observed correlations remain clearly detectable and are consistently well-approximated by Gaussian statistics. This confirms that the two sources can be reliably synchronized not only in controlled laboratory conditions but also under real-world operating constraints.
Time synchronization is crucial in the architecture of modern communication networks, supporting numerous high-stakes applications like financial transactions, autonomous vehicle control, and data center operations. While traditional time synchronization protocols, specifically the Network Time Protocol (NTP) and Precision Time Protocol (PTP), are reliable for various applications, they fall short in scenarios requiring ultra-high precision and resilience. To address these limitations, this paper provides a comprehensive comparative analysis of two emerging quantum technologies, namely Time-Correlated Entangled Photons (TCEP) and Optical Lattice Clocks (OLC). Using Monte Carlo simulations, we examined the synchronization in terms of the accuracy of these technologies under various noise conditions, revealing that while TCEP works perfectly in low-noise environments, its efficacy diminishes significantly with increasing noise levels. On the contrary, OLCs demonstrate consistent performance across various noise levels, making them more versatile for diverse application scenarios. This study is foundational for integrating quantum technologies in time synchronization for communication networks and sheds light on their merits and challenges. Our findings open new avenues for research in scalability, environmental resilience, and the development of hybrid quantum-classical timekeeping systems. Integrating quantum-enhanced time synchronization into a communication network will be the key step for achieving full-fledged Quantum Internet and quantum-enhanced communication networks.
Quantum clock synchronization (QCS) aims to establish a shared temporal reference between distant nodes by exploiting uniquely quantum phenomena such as entanglement, single-photon interference, and quantum correlations. In contrast to classical synchronization and time-transfer techniques, which are limited by signal propagation delays, atmospheric disturbances, and oscillator drift, QCS protocols offer the potential to surpass classical precision bounds and enhance resilience against adversarial manipulations. As precise and secure time synchronization underpins distributed quantum networks, navigation systems, and emerging quantum Internet infrastructures, understanding QCS principles, capabilities, and implementation challenges has become increasingly important. This survey provides a unified and critical overview of the rapidly growing QCS research landscape, highlighting fundamentals, protocol types, enabling resources, performance constraints, security considerations, and practical implementations of QCS. We first introduce the theoretical underpinnings of QCS, including entanglement-assisted time transfer, Hong-Ou-Mandel interference-based synchronization, and quantum slow-clock transport. We then categorize the main QCS protocols, ranging from ticking-qubit and entanglement-based schemes to time-of-arrival correlation methods, conveyor-belt synchronization, and quantum-enhanced two-way time transfer. This organization clarifies the relationships between protocol families and their achievable precision advantages over classical methods. Key quantum resources such as spontaneous parametric down-conversion-based entangled photon pairs, Greenberger–Horne–Zeilinger and W multipartite states, squeezed and frequency-entangled light, quantum frequency combs, and quantum memories are reviewed in the context of scalability and robustness. Practical limitations arising from decoherence, channel loss, dispersion, detector timing jitter, and relativistic effects are examined alongside emerging noise mitigation strategies. Security aspects—including eavesdropping on time correlations, intercept-resend attacks, and adversarial delay manipulation—are evaluated with respect to trusted and untrusted network architectures. Recent proof-of-concept demonstrations in fiber networks, free-space optical links, chip-integrated photonic systems, and satellite-ground platforms are summarized, followed by discussions of open challenges toward precise and secure global quantum-synchronized networks.
Quantum clock synchronization (QCS) aims to establish a shared temporal reference between distant nodes by exploiting uniquely quantum phenomena such as entanglement, single-photon interference, and quantum correlations. In contrast to classical synchronization and time-transfer techniques, which are limited by signal propagation delays, atmospheric disturbances, and oscillator drift, QCS protocols offer the potential to surpass classical precision bounds and enhance resilience against adversarial manipulations. As precise and secure time synchronization underpins distributed quantum networks, navigation systems, and emerging quantum Internet infrastructures, understanding QCS principles, capabilities, and implementation challenges has become increasingly important. This survey provides a unified and critical overview of the rapidly growing QCS research landscape, highlighting fundamentals, protocol types, enabling resources, performance constraints, security considerations, and practical implementations of QCS. We first introduce the theoretical underpinnings of QCS, including entanglement-assisted time transfer, Hong-Ou-Mandel interference-based synchronization, and quantum slow-clock transport. We then categorize the main QCS protocols, ranging from ticking-qubit and entanglement-based schemes to time-of-arrival correlation methods, conveyor-belt synchronization, and quantum-enhanced two-way time transfer. This organization clarifies the relationships between protocol families and their achievable precision advantages over classical methods. Key quantum resources such as spontaneous parametric down-conversion-based entangled photon pairs, Greenberger-Horne-Zeilinger and W multipartite states, squeezed and frequency-entangled light, quantum frequency combs, and quantum memories are reviewed in the context of scalability and robustness.