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Proceedings of the ... annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting/Proceedings of a meeting - Precise Time and Time Interval (PTTI) Systems and Applications Meeting

Demonstration of Real-Time Precision Optical Time Synchronization in a True Three-Node Architecture

Kyle W. Martin, Matthew S. Bigelow, Nader Zaki, Benjamin Stuhl, Nolan Matthews, John Elgin, Kimberly Frey

Published Feb 15, 2024• 0 citations
Publisher

Abstract

Multi-node optical clock networks will enable future studies of fundamental physics and enable applications in quantum and classical communications as well as navigation and geodesy. We implement the first ever multi-node optical clock network with real-time, relative synchronization over free-space communication channels and precision on the order of 10 fs, realized as a three-node system in a hub-and-spoke topology. In this paper we describe the system and its performance, including a new, independent, out-of-loop verification of two-way optical time synchronization.

DOI
10.33012/2024.19591

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Multi-node optical clock networks will enable future studies of fundamental physics and enable applications in quantum and classical communications as well as navigation and geodesy. We implement the first ever multi-node optical clock network with real-time, relative synchronization over free-space communication channels and precision on the order of 10 fs, realized as a three-node system in a hub-and-spoke topology. In this paper we describe the system and its performance, including a new, independent, out-of-loop verification of two-way optical time synchronization.
Abstract Multi-node optical clock networks will enable future studies of fundamental physics and enable applications in quantum and classical communications as well as navigation and geodesy. We implement the first ever multi-node optical clock network with real-time, relative synchronization over free-space communication channels and precision on the order of 10 femtoseconds, realized as a three-node system in a hub-and-spoke topology. In this paper we describe the system and its performance, including a first ever measurement of precision optical time synchronization between nodes with no direct communication link or causal feedback relationship.
The continued remarkable advances in optical clocks and oscillators has led to a parallel strong development of optical clock networks [1]. Such networks have the potential to support a wide range of applications from basic time/frequency dissemination, to clock-based geodesy, to tests of general relativity [1]. To support optical clocks/oscillators at their highest accuracy and precision, these networks must rely on optical signals between the nodes to carry the timing signals. Networks based on the reciprocal transmission across fiber optics have allowed frequency comparisons between remote clocks across Europe and Japan [2, 3]. However, there are many potential applications, both terrestrial and space-based, for which the clocks will not be connected via an optical fiber and the signals must be sent across free space optical links. To this end, we have been developing techniques for the free-space transfer of time and frequency [4-6]. Moreover, rather than pursuing only frequency comparison, we have developed an approach that allows time comparison and even real-time synchronization between nodes. Such a capability can support the wide variety of proposed applications of mobile optical clock networks.
SummaryHigh-precision time comparison and synchronization is required with the increasing application of the optical clocks in precise navigation, gravitational sensing, and relativity experiments. However, previous works at femtosecond-level time synchronization did not extend beyond dozens of kilometers. Here we demonstrate real-time synchronization of two optical clocks over a turbulent 113-km free space link via optical two-way time and frequency transfer. Despite optical power attenuation of 80dB due to atmospheric turbulence, the time deviation between two synchronized time scales is below 50 fs over a 15-hour measurement period, and a stability of less than 1.6 × 10−19at 10,000 s is achieved through an out-of-loop time-frequency transfer link. The ability to synchronize two distant optical clocks can support future precision navigation, and it also provides technical foundation for the future femtosecond clock synchronization over ground-to-satellite free-space paths.
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.
The ability to measure, hold and distribute time with high precision and accuracy is a foundational capability for scientific exploration. Beyond fundamental science, time synchronization is an indispensable feature of public and private communication, navigation and ranging, and distributed sensing, amongst others technological applications. We propose the implementation of a quantum network of satellite- and ground-based clocks with the ability to implement Quantum Clock Synchronization to picosecond accuracy. Implementation of the proposed QCS network offers a double advantage: (1) a more accurate, robust, and secure time synchronization network for classical applications than currently possible, and (2) a resource to fulfill the much more stringent synchronization requirements of future quantum communication networks.
The use of optical clocks or oscillators in future ultraprecise navigation, gravitational sensing, coherent arrays, and relativity experiments will require time comparison and synchronization over terrestrial or satellite free-space links.Here, we demonstrate full unambiguous synchronization of two optical time scales across a free-space link.The time deviation between synchronized time scales is below 1 fs over durations from 0.1 to 6500 s, despite atmospheric turbulence and kilometer-scale path length variations.Over 2 days, the time wander is 40 fs peak to peak.Our approach relies on the two-way reciprocity of a single-spatial-mode optical link, valid to below 225 attoseconds across a turbulent 4-km path.This femtosecond level of time-frequency transfer should enable optical networks using state-of-the-art optical clocks or oscillators.