Quantum entanglement serves as a vital resource for quantum information technology. The practical application of entanglement distribution necessitates stable time synchronization between remote parties to preserve quantum correlation. However, synchronization in practical systems, particularly in quantum networks, is required not only to provide high stability but also to minimize additional optical resource requirements and avoid interference with quantum photons, thereby ensuring deployment flexibility. Here, we propose a sequential sliced time synchronization scheme that enables high-stability synchronization through post-processing, without requiring additional optical resources. This scheme employs a global navigation satellite system clock, without atomic clock assistance, for coarse identification of coincidence events, followed by fine tracking and compensation of timing offset drift by partitioning the event stream into sequential slices. Experimentally, we demonstrate compatibility with entanglement-based quantum key distribution over 100.8 km of single-mode fiber with a 19.52 dB loss, achieving a finite-size secure key rate of 496 bps at a residual synchronization error of 0.27 ns. These results demonstrate that high-stability synchronization for metropolitan-scale entanglement distribution is achievable using a global navigation satellite system without atomic clock assistance, facilitating scalable deployment of quantum networks.
Quantum entanglement serves as a vital resource for quantum information technology. The practical application of entanglement distribution necessitates stable time synchronization between remote parties to preserve quantum correlation. However, synchronization in practical systems, particularly in quantum networks, is required not only to provide high stability but also to minimize additional optical resource requirements and avoid interference with quantum photons, thereby ensuring deployment flexibility. Here, we propose a sequential sliced time synchronization scheme that enables high-stability synchronization through post-processing, without requiring additional optical resources. This scheme employs a global navigation satellite system clock, without atomic clock assistance, for coarse identification of coincidence events, followed by fine tracking and compensation of timing offset drift by partitioning the event stream into sequential slices. Experimentally, we demonstrate compatibility with entanglement-based quantum key distribution over 100.8 km of single-mode fiber with a 19.52 dB loss, achieving a finite-size secure key rate of 496 bps at a residual synchronization error of 0.27 ns. These results demonstrate that high-stability synchronization for metropolitan-scale entanglement distribution is achievable using a global navigation satellite system without atomic clock assistance, facilitating scalable deployment of quantum networks.
Accurate timekeeping is essential for scientific and technological advancements, particularly in areas of communication, networking, navigation, and high precision measurements. While many methods of time resolution are already established using classical resources, they fail to combine high precision outcomes with large-scale implementations. Additionally, numerous quantum networking architectures using satellite-assisted methods have demonstrated quantum communication on scales exceeding ground-based methods. For these reasons, we propose the use of a time synchronization method by which pairs of highly time-correlated photons are exchanged between clocks on satellites and clocks on Earth, al- lowing users to reconstruct the time offsets between their clocks. This is known as Quantum Clock Synchronization (QCS). We analyze global time synchronization, via numerical simulation, using a network of satellites equipped with quantum resources, such as entangled photon sources (SPDC) and single photon detectors. We find that for satellites in low-earth orbits, a configuration of 50 evenly-space satellites provides constant visibility and sub-nanosecond time synchronization to any location on Earth. Due to practical constraints, clocks available for use on satellites are often of lesser quality than those available for ground use. However, by allowing the satellites in our network to synchronize among themselves, we overcome this limitation, forming a highly precise “master clock” capable of global, high precision time synchronization. We then examine the amount of quantum security provided by polarization-entangled photon sources as a potential way to protect an experimental implementation of QCS from attacks such as signal spoofing. To do this, we identify regions on Earth where Bell violations between exchanged entangled photons can ensure quantum security at various confidence vii levels. Combined with similar insights gained from determining the network scale of our QCS protocol at the single-link level, we demonstrate the potential for highly precise, global, and potentially quantum-secure clock synchronization. Finally, we modify our QCS protocol to support frequency, or tick-rate, offsets in addition to constant time offsets. We motivate this by presenting some relativistic effects that can introduce frequency differences between clocks in space and on Earth. Provided that relative velocities between clocks are corrected for, we identify constant and variable clock defects through numerical simulations.
We present the implementation of a time synchronization protocol as a part of an experimentally deployed entanglement-based quantum key distribution (QKD) link. The system is deployed over 48 km of optical fibers across the Métropole Côte d'Azur and enables secret cryptographic key exchange between two remote users, with an average rate of 7 kbps. We exploit the time correlation of paired photons generated by a high-quality source of energy-time entanglement implemented in the QKD link to synchronize two rubidium clocks located at the end stations. The level of stability achieved guarantees a time offset between the clocks under 12 ps at all time. We also show that this protocol requires less hardware than a typical synchronization protocol for QKD that would distribute a reference clock signal between the users.
Precise timing synchronization is essential for distributed quantum networking, enabling entanglement distribution, quantum teleportation, and entanglement swapping across remote nodes. Existing synchronization architectures rely on dedicated timing-distribution infrastructure, most notably White Rabbit networks, which constrain topology, scalability, and deployment in free-space and satellite environments. Here we demonstrate link-free synchronization of quantum network nodes using independently operating miniature rubidium atomic clocks and computational post-processing. We validate the approach on a deployed metropolitan-scale telecom fiber network spanning three geographically separated nodes. Following drift correction, atomic-clock-based synchronization achieves timing performance approaching that of a White Rabbit benchmark and remains stable over continuous 8-hour operation. As a stringent test of quantum-network functionality, we observe Hong-Ou-Mandel interference across spatially separated nodes with visibility exceeding 70%, statistically equivalent to that obtained using dedicated White Rabbit timing links. To the best of our knowledge, this represents the first observation of quantum interference across a deployed metropolitan-scale telecom fiber network synchronized entirely without dedicated timing-transfer infrastructure. These results establish atomic-clock-based synchronization as a scalable, topology-independent alternative to conventional timing-distribution architectures and a practical pathway toward terrestrial, airborne, and space-based quantum networks where dedicated timing links are unavailable.
Clock synchronization is critical for maintaining low error rates in quantum key distribution. Here, we describe how a frequency mismatch between the transmitter and receiver clocks affects the quantum bit error rate in quantum key distribution, and derive from this model a simple synchronization algorithm together with clock stability requirements for practical operation. Our algorithm continuously compensates for both frequency mismatch and time-offset fluctuations directly from detection timestamps. It does not require a dedicated synchronization channel or auxiliary qubit sequence, converges from a large frequency mismatch within approximately one second of photon acquisition, and remains effective in low-photon-count regimes (more than 30 dB of channel loss) using standard hardware. We validate our approach by demonstrating successful key exchange over 100 km of fiber and continuous operation over 24 hours in a 16 km metropolitan network using commercial systems, with performance equivalent to using a service channel for clock synchronization.
Drone-based entanglement distribution provides full spatiotemporal coverage for quantum networks, enabling quantum key distribution (QKD) in dynamic environments. The security of QKD fundamentally depends on high-fidelity quantum state measurements, for which high-precision clock synchronization is indispensable, as timing jitter is inversely correlated with quantum state fidelity. However, drone-based clock synchronization is constrained by SWaP (Size, Weight, and Power) limitations and dynamic mobility effects. Here, we propose a synchronization protocol for drone-based entanglement distribution, leveraging nanosecond-accurate Global Navigation Satellite System (GNSS) timing and entanglement-based timing correction to overcome SWaP constraints. Experimental results demonstrate 24 ps RMS synchronization in simulated free-space quantum channels with distance dynamics, without requiring precision reference clock. Our protocol enables drone-based entanglement distribution, paving the way for seamless wide-area and local-area quantum internet.
We propose a satellite-based scheme to perform clock synchronization between ground stations spread across the globe using quantum resources. We refer to this as a quantum clock synchronization (QCS) network. Through detailed numerical simulations, we assess the feasibility and capabilities of a near-term implementation of this scheme. We consider a small constellation of satellites equipped only with modest resources. These include quantum devices such as spontaneous parametric down conversion sources, avalanche photodetectors, and moderately stable on-board clocks such as chip-scale atomic clocks. In our simulations, the various performance parameters describing the hardware have been chosen such that they are either already commercially available or require only moderate advances. We conclude that with such a scheme, establishing a global network of ground-based clocks synchronized to subnanosecond level (up to a few picoseconds) of precision would be feasible. Such QCS satellite constellations would form the infrastructure for a future quantum network, able to serve as a globally accessible entanglement resource. At the same time, our clock synchronization protocol provides the subnanosecond level synchronization required for many quantum networking protocols, and thus can be seen as adding an extra layer of utility to quantum technologies in the space domain designed for other purposes.