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Optics Express

Quantum two-way time transfer over a 250 km direct fiber-optic link

Bingke Shi, Huibo Hong, Xiao Xiang, Runai Quan, Yiyu Liu, Tao Liu, Shougang Zhang, Ruifang Dong

Published Nov 11, 2024• 8 citations
Publisher

Abstract

Utilizing energy-time entangled biphotons as the resource, quantum two-way time transfer (Q-TWTT) has shown superior transfer stability far below 1 picosecond level in numerous experimental demonstrations. In this article, we provide further insights into comparing this method with the classical time transfer method based on intensity modulation-direct detection (IM-DD) regarding their signal-to-noise ratio (SNR) performance against backscattering noise in long-distance fiber transmission. The exceptional SNR of Q-TWTT indicates its potential to pioneer new avenues for overcoming the transmission distance limitation in single-span fiber. In light of this, a groundbreaking achievement of Q-TWTT is demonstrated over a remarkable 250 km-long direct fiber-optic link. Despite a total transmission loss of 55 dB, time transfer stability of 4.9 ps at an averaging time of 40 s and 0.6 ps at 10240 s is achieved. This marks a significant advancement in the experimental demonstration of long-haul fiber quantum time transfer, which also offers a highly compatible and potentially quantum secure solution for constructing large-scale quantum networks.

DOI
10.1364/oe.536691

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Utilizing energy-time entangled biphotons as the resource, quantum two-way time transfer (Q-TWTT) has shown superior transfer stability far below 1 picosecond level in numerous experimental demonstrations. In this article, we provide further insights into comparing this method with the classical time transfer method based on intensity modulation-direct detection (IM-DD) regarding their signal-to-noise ratio (SNR) performance against backscattering noise in long-distance fiber transmission. The exceptional SNR of Q-TWTT indicates its potential to pioneer new avenues for overcoming the transmission distance limitation in single-span fiber. In light of this, a groundbreaking achievement of Q-TWTT is demonstrated over a remarkable 250 km-long direct fiber-optic link. Despite a total transmission loss of 55 dB, time transfer stability of 4.9 ps at an averaging time of 40 s and 0.6 ps at 10240 s is achieved. This marks a significant advancement in the experimental demonstration of long-haul fiber quantum time transfer, which also offers a highly compatible and potentially quantum secure solution for constructing large-scale quantum networks.
Abstract As the superiority of quantum two-way time transfer (Q-TWTT) has been proved convincingly over fiber links, its implementation on free-space links becomes an urgent need for remote time transfer expanding to the transcontinental distance. In this paper, the first Q-TWTT experimental demonstration over a hybrid link of 2 km-long turbulent free-space and 7 km-long field fiber is reported. Despite the significant loss of ∼30 dB and atmospheric turbulence, reliable time transfer performance lasting for overnights has been realized with time stability in terms of time deviation far below 1 picosecond. This achievement shows the good feasibility of quantum-enhanced time transfer in the space-ground integrated optical links and nicely certifies the capability of Q-TWTT in comparing and synchronizing the state-of-the-art space microwave atomic clocks.
In this study, we conducted experiments using a single energy-time entangled biphoton source to compare the performance of quantum two-way time transfer (Q-TWTT) and quantum round-trip time transfer (Q-RTTT) under consistent spatiotemporal conditions. By conducting experiments with different fiber links of 11.3 km, 22.4 km, and 55.6 km, while ensuring uniform photon counts received by the single-photon detectors, we measured standard deviations (SDs) and stabilities of the time offsets. The measured SDs for Q-TWTT and Q-RTTT were 0.46 ps and 0.65 ps over the 11.3 km fiber, 1.14 ps and 1.3 ps over the 22.4 km fiber, 3.98 ps and 4.39 ps over the 55.6 km fiber, respectively. These results show good agreement with theoretical predictions, and the smaller SDs for Q-TWTT can be directly attributed to protocol-specific factors related to system symmetry. The long-term time stabilities of Q-TWTT and Q-RTTT were evaluated in terms of time deviation (TDEV). At an average time of 12680 s, the measured TDEVs were 0.49 ps and 0.63 ps for the 11.3 km fiber, 0.59 ps and 0.7 ps for the 22.4 km fiber, 1.01 ps and 1.36 ps for the 55.6 km fiber, respectively. The results validate that Q-TWTT exhibits superior time transfer performance compared to Q-RTTT, highlighting the advantages of Q-TWTT in practical 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.
As a new approach to realizing high-precision time synchronization between remote time scales, quantum two-way time transfer via laboratory fiber link has shown significant enhancement of the transfer stability to several tens of femtoseconds. To verify its great potential in practical systems, the field test in long-haul installed fiber optic infrastructure is required to be demonstrated. In this article, we implement the two-way quantum time transfer over a 103 km urban fiber link. A time transfer stability of 3.67 ps at 10 s and 0.28 ps at 40 000 s has been achieved, despite the large attenuation of 38 dB leading to fewer than 40 correlated events per second. This achievement marks the first successful step of quantum two-way time transfer in the task of high-precision long-distance field transfer systems.
Quantum two-way time transfer (Q-TWTT) leveraging energy-time entangled biphotons has achieved sub-picosecond stability but faces fundamental distance limitations due to the no-cloning theorem's restriction on quantum amplification. To overcome this challenge, we propose a cascaded Q-TWTT architecture employing relay stations that generate and distribute new energy-time entangled biphotons after each transmission segment. Theoretical modeling reveals sublinear standard deviation growth (merely N increase for N equidistant segments), enabling preservation of sub-picosecond stability over extended distances. We experimentally validate this approach using a three-station cascaded configuration over 200 km fiber segments, demonstrating strong agreement with theory. Utilizing independent Rb clocks at end and relay stations with online frequency skew correction, we achieve time stabilities of 3.82 ps at 10 s and 0.39 ps at 5120 s. The consistency in long-term stability between cascaded and single-segment configurations confirms high-precision preservation across modular quantum networks. This work establishes a framework for long-distance quantum time transfer that surpasses the no-cloning barrier, providing a foundation for future quantum-network timing infrastructure.
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.