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arXiv:2109.00784 · Optics Express

Implementation of field two-way quantum synchronization of distant clocks across a 7 km deployed fiber link

Runai Quan, Huibo Hong, Wenxiang Xue, Honglei Quan, Wenyu Zhao, Xiao Xiang, Yuting Liu, Mingtao Cao, Tao Liu, Shougang Zhang, Ruifang Dong

Published Mar 7, 2022• 30 citations
arXivHTMLPDF

Abstract

The two-way quantum clock synchronization has been shown to provide femtosecond-level synchronization capability and security against symmetric delay attacks, thus becoming a prospective method to compare and synchronize distant clocks with enhanced precision and safety. In this letter, a field test of two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber is implemented by using time-energy entangled photon-pair sources. Limited by the intrinsic frequency stability of the Rb clock, the achieved time stability at 30 s is measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology to build frequency syntonization between the separated clocks, the limit set by the intrinsic frequency stability of the Rb clock is overcome. A significantly improved time stability of 1.9 ps at 30 s is achieved, which is mainly restrained by the low number of acquired photon pairs due to the low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of the two-way quantum clock synchronization method for promoting field applications.

DOI
10.1364/oe.451172

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The two-way quantum clock synchronization has been shown to provide femtosecond-level synchronization capability and security against symmetric delay attacks, thus becoming a prospective method to compare and synchronize distant clocks with enhanced precision and safety. In this letter, a field test of two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber is implemented by using time-energy entangled photon-pair sources. Limited by the intrinsic frequency stability of the Rb clock, the achieved time stability at 30 s is measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology to build frequency syntonization between the separated clocks, the limit set by the intrinsic frequency stability of the Rb clock is overcome. A significantly improved time stability of 1.9 ps at 30 s is achieved, which is mainly restrained by the low number of acquired photon pairs due to the low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of the two-way quantum clock synchronization method for promoting field applications.
The two-way quantum time transfer method has been proposed and experimentally demonstrated for its potential enhancements in precision and better guarantee of security. To further testify its advantage in practical applications, the applicable direct transmission distance as well as the achievable synchronization precision between independent time scales is of great interest. In this paper, an experiment on two-way quantum time transfer has been carried out over a 50 km long fiber link. With the common clock reference, a short-term stability of 2.6 ps at an averaging time of 7 s and a long-term stability of 54.6 fs at 57300 s were obtained. With independent clock references, assisted by microwave frequency transfer technology, the achieved synchronization showed almost equal performance and reached a stability of 89.5 fs at 57300 s. Furthermore, the spectral consistency of the utilized entangled photon pair sources has been studied concerning its effect on the transfer accuracy and long-term stability. The results obtained have promised a bright future of the two-way quantum time transfer for realizing high-precision time synchronization on metropolitan area fiber links.
Fiber-optic Quantum clock synchronization has been proposed and experimentally demonstrated for its potential enhancements in precision and better guarantee of security. To further prove its advantage in practical applications, it is necessary to test its synchronization performance between clocks apart for at least intracity optical fiber distance. In this paper, we report a fiber-optic two-way quantum clock synchronization experiment with the distance reaching 50 km. With the common reference clock, a synchronization stability of 54.6 fs at 57300 s and an accuracy of 1.3 ps±36.6 ps have been achieved. With independent reference clocks, comparable performance has also been achieved combined with the microwave frequency transfer technology, which shows a stability of 89.5 fs at 57300 s. These results nicely showcase the feasibility of the two-way quantum clock synchronization to provide femtosecond-scale synchronization precision over intracity optical fiber links.
In this Letter, we report a prototype system that realizes the complete functionality for quantum two-way time transfer, comparison, and synchronization between two integrated terminals. The synchronization performance was tested over a 50-km spooled fiber link. With the common frequency reference, the time deviation was measured as 0.45 ps at an averaging time of 104 s, which is limited by the system's hardware and determines the minimum achievable synchronization stability. By employing an open-loop fiber-optic microwave frequency transfer in combination with the technique of dynamically identifying and steering the time offset between the terminals, a synchronization stability of 1.26 ps at 104 s was achieved. Further utilizing the grey prediction model to correct the time offset data, the synchronization stability was significantly improved to 0.69 ps at 104 s, showing its potential to enhance the synchronization performance. This report marks the development of a utility quantum two-way clock synchronization system. The ongoing exploration of advanced time-offset adjustment strategies to attain synchronization stability significantly below 1 ps is poised to yield invaluable benefits for future applications.
Based on frequency entangled sources and arrival time measurements, a best time synchronization stability result of fewer than 100 fs has been reported over 20km fiber links, verifying the superiority of quantum time synchronization. In this paper, a two-way quantum time transfer over 50 km fiber links with the same frequency standard was implemented, with a short-term stability of 2.63 ps at an averaging time of 18 s and a long-term stability of 79.2 fs at 73700 s. The system accuracy in terms of the 50km fiber transfer length is measured as 524.75 ps, the agreement of which with the theoretical simulation illustrates that improving the spectral consistency of the two entangled sources will significantly improve the accuracy. This result shows that the fiber-optic two-way quantum time transfer can be successfully extended to a metropolitan fiber link distance of around 50 km or longer, not only maintaining the transfer stability well below picosecond but also promising further improvements in synchronization accuracy.
The quantum clock synchronization based on frequency-correlated photon pairs and HOM interference has shown femtosecond-level precision and great application prospect in numerous fields depending on high-precision time-frequency signals. Due to the difficulty of achieving stable HOM interference fringe after long-distance fiber transmission, this scheme of synchronization is hampered from long-haul field application. Utilizing segmented fibers instead of a single long-length fiber, we achieved the stable observation of the two-photon interference of the lab-developed broadband frequency-correlated photon pairs after 20 km-long fiber transmission, without employing the auxiliary phase stabilization method. Referenced to this interference fringe, the balance of the two fiber arms is achieved with a long-term stability of 20 fs. The HOM-interference-based synchronization over a 20-km fiber link is thus demonstrated, and a minimum stability of 74 fs has been reached at 48 000 s. This result not only provides a simple way to stabilize the fiber-optic two-photon interferometer for long-distance quantum communication systems but also makes a great stride forward in extending the quantum-interference-based synchronization scheme to the long-haul field applications.
Quantum clock synchronization schemes utilizing frequency-entangled pulses have flourished for their potentially superior precision to the classical protocols. In this Letter, a new experimental record based on the second-order quantum interference algorithm is reported, to the best of our knowledge. The synchronization accuracy between two parties separated by a 6 km fiber coiling link, which is evaluated by the time offset shift relative to that with the fibers removed, has been measured to be 13±1 ps. The stability in terms of time deviation (TDEV) of 0.81 ps at an averaging time of 100 s has been achieved. The long-term synchronization stability is seen determined by the measurement device, and a minimum stability of 60 fs has been reached at 25,600 s. Furthermore, for the first time to the best of our knowledge, we quantify the performance of this quantum synchronization scheme, and very good agreements with the experimental results have been achieved. According to the quantum simulation, further improvements for both the synchronizing stability and accuracy can be expected.