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arXiv:1602.06371 · Scientific Reports

Demonstration of quantum synchronization based on second-order quantum coherence of entangled photons

Runai Quan, Yiwei Zhai, Mengmeng Wang, Feiyan Hou, Shaofeng Wang, Xiao Xiang, Tao Liu, Shougang Zhang, Ruifang Dong

Published Jul 25, 2016• 62 citations
arXivHTMLPDF

Abstract

Based on the second-order quantum interference between frequency entangled photons that are generated by parametric down conversion, a quantum strategic algorithm for synchronizing two spatially separated clocks has been recently presented. In the reference frame of a Hong-Ou-Mandel (HOM) interferometer, photon correlations are used to define simultaneous events. Once the HOM interferometer is balanced by use of an adjustable optical delay in one arm, arrival times of simulta- neously generated photons are recorded by each clock. The clock offset is determined by correlation measurement of the recorded arrival times. Utilizing this algorithm, we demonstrate a proof-of-principle experiment for synchronizing two clocks separated by 4 km fiber link. A minimum timing stability of 0.44 ps at averaging time of 16000 s is achieved with an absolute time accuracy of 73.2 ps. The timing stability is verified to be limited by the correlation measurement device and ideally can be better than 10 fs. Such results shine a light to the application of quantum clock synchronization in the real high-accuracy timing system.

DOI
10.1038/srep30453

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We present an algorithm for synchronizing two clocks based on second‐order quantum interference between entangled photons generated by parametric down‐conversion. The procedure is distinct from the standard Einstein two‐way clock synchronization method in that photon correlations are used to define simultaneous events in the frame of reference of a Hong‐Ou‐Mandel (HOM) interferometer. Once the HOM interferometer is balanced, by use of an adjustable optical delay in one arm, arrival times of simultaneously generated photons are recorded by each clock. Classical information on the arrival times is sent from one clock to the other, and a correlation of arrival times is done to determine the clock offset.
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.
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.
Abstract Establishing and maintaining a common time reference across spatially separated devices is a prerequisite for networked quantum experiments and secure communications. Classical two-way timing protocols such as network time protocol or precision time protocol are vulnerable to asymmetric channel delays and cannot provide the picosecond-level precision demanded by quantum repeater networks. We propose and numerically evaluate a quantum-enhanced clock synchronization protocol based on attenuated weak coherent pulses (WCPs) and bidirectional Hong–Ou–Mandel (HOM) interferometry. Our simulations assume telecom-band photons (1550 nm) with a temporal width of 10.0 ns, a repetition rate of f = 10 MHz, effective mean photon number µ = 1.0, detector efficiency η = 85 % , detector timing jitter of 150 ps and channel loss 0.2 dB km −1 . We simulate that sub-nanosecond clock-offset accuracy and precision can be achieved under these operating conditions. This work demonstrates that high-repetition-rate WCPs combined with HOM interference can provide flexible and secure quantum clock synchronization at sub-nanosecond precision.