| High-precision clock synchronization plays an important role in advanced scientific researches and technical applications such as metrology,deep space exploration,global navigation and positioning,and gravitational wave detection.Since the fact that: the construction of time-frequency system has been included into the development plan of the 12 th Five-Year Plan,and Beidou satellite navigation system and national PNT(time,positioning and navigation)system will be implemented as well;a high-precision timefrequency cabinet will be developed and operated at the China Space Station in 2022;the National Natural Science Foundation of China has established a major research program for precision measurement physics,all of the above planned tasks require the development of clock synchronization techniques which are superior to picoseconds.Quantum clock synchronization is a crossover field of quantum optics,quantum information and time-frequency.Using optical signals with quantum properties such as entanglement and compression and highly sensitive quantum detection technique,the accuracy of clock synchronization can be greatly improved.By utilizing the frequency entanglement,the influence of dispersion in propagation path can be eliminated in corresponding quantum clock synchronization protocol.With the unique characteristics of quantum entangled sources,it is also possible to develop a secure clock synchronization algorithm.In view of the above properties,quantum clock synchronization is reckoned as one of the important directions in the field of quantum metrology in this century.It is also considered to be a forward-looking clock synchronization technology with higherprecision in the future.Coincident-frequency entangled source is an important physical source in quantum clock synchronization due to its high temporal resolution and dispersion cancellation property.In this paper,coincident-frequency entangled biphotons is utilized as the source of quantum clock synchronization protocol.Based on the experimentally generated frequency entangled photons,a sub-picosecond quantum clock synchronization has been realized in a kilometer-scale fiber in the laboratory for the first time.A detailed analysis of the performance of quantum clock synchronization is stated,and according to the analysis,an optimized system has been implemented.The synchronization stability up to tens of femtoseconds at a 6 km fiber spools has been achieved.A nonlocal time offset identification algorithm is a prerequisite technique for the development of field quantum clock synchronization.In this paper,a high-precision time offset identification algorithm based on event timers is presented,which provides the feasibility of realizing sub-picosecond clock synchronization accuracy.The main contents of this paper are as follows:1.Based on Hong Ou Mandel(HOM)second-order quantum coherence between frequency entangled photons that are generated by parametric down conversion,a proof-ofprinciple experiment for synchronizing two clocks spatially separated has been designed and established in this paper,including the path-balance control system and time offset measurement part.In the reference frame,a HOM interferometer is used to define the path-length balance of the two sites.Once the path-length is balanced,the clock offset is determined by coincidence measurement of the recorded arrival times.By utilizing 1 PPS signal as the input,the characterization of clock offset measurement part is evaluated.The performance of quantum clock synchronization scheme has been quantified theoretically,where both the non-degeneracy of biphotons and the second-order dispersion of fiber links contributed to the synchronization accuracy,and the standard deviations of clock offset measurement were affected by wavepacket broadening of biphotons and the coincidence counting rate.2.For the first time to the best of our knowledge,a subpicosecond quantum clock synchronization result was reported.Based on semiconducting single photon detectors,a timing stability of 1.5 ps at averaging time of 1000 s,and a minimum timing stability of0.44 ps at averaging time of 16000 s are achieved with an absolute accuracy of 73.2 ps between two clocks separated by 4 km fiber coiling.By utilizing superconductive nanowire single photon detectors,the synchronization accuracy between two parties separated by a6 km fiber coiling link has been measured to be 13 ps.The stability in terms of time deviation(TDEV)of 0.81 ps at averaging time of 100 s has been achieved,and a minimum stability of 60 fs has been reached at 25600 s.The above results show good agreement with theoretical simulations.3.In order to solve the problem that existed in traditional coincidence device,and execute a field quantum clock synchronization,a nonlocal time offset identification algorithm,which realizes well-performed functionality analogous to a dedicated coincidence hardware with a tunable resolution down to 1 ps,has been designed and optimized based on event timers.By utilizing event timers to time tag and record the arrival time of entangled photons,a second-order correlation operation is implemented to realize the coincidence distribution of the photon pairs.With a common time scale reference based on our algorithm,the measured time offset between two timing signals detected nonlocally has achieved a standard deviation of 0.79 ps.An analysis of the attainable precision and the potential applications of the algorithm have also been stated. |
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