Characterization Of Frequency Entangled Source And The Application Investigation In Conveyor-belt Quantum Clock Synchronization

The long-distance and high-precision clock synchronization system plays an im-portant role in many areas,such as navigation and positioning,gravitational wave detec-tion and basic scientific research.The accuracy of the traditional clock synchronization method is limited by the shot noise limit of classical measurement.In the quantum system,the entangled photon pair can break through the shot noise limit due to its en-tanglement characteristics and reach the principle of natural physics-the Heisenberg limit of quantum mechanics.Therefore,quantum clock synchronization using quantum entangled sources has a promising application in high precision clock synchronization systems.Using frequency entangled source with quantum properties and highly sensitive quantum detection technology,the accuracy of clock synchronization can be greatly im-proved.Therefore,quantum time synchronization is reckoned as one of the important directions in time-frequency systems.A frequency entangled biphoton source generated from a type-? phase-matched PPKTP crystal under a femtosecond pulse pump.And the spectral indistinguishability and the degree of frequency entanglement of biphoton source can be simultaneously measured based on a Mach-Zehnder interferometric quan-tum coincidence setup.In the meantime,the quantum measurement of Sagnac effect is achieved using frequency entangled source.The conveyor-belt quantum time synchro-nization protocol is theoretically extended and experimentally studied by combining the Sagnac effect.The main research contents and research results are as follows:1.We optimized the generating device of the frequency entangled source,and an-alyzed the optimal focusing size of the pump pulse incident to the PPKTP crystal under the condition of a certain output power.The loss and dispersion of the entangled photon pair in the optical fiber transmission path are analyzed,and the dispersion compensation function of the dispersion compensation fiber is verified.We compared the performance of the semiconductor single-photon detector and the superconducting nanowire single-photon detector,the detection efficiency of the superconducting nanowire single-photon detector is up to 50%,the timejitter is 79ps,and the dark count is 200Hz,which is greatly improved compared with the detection efficiency of the semiconductor single-photon detector 10%,the time jitter of 249ps and the dark count of 2.2kHz.2.It has been demonstrated that for a frequency entangled photon pair source gen-erated under pulsed pump and type-? SPDC progress,both the spectral indistinguisha-bility and the degree of frequency entanglement can be simultaneously measured based on a MZ interferometer.We derive the temporal distribution of such a biphoton source and its relevant temporal entanglement parameter The results show that,due to the inherent group delay difference between the generated polarization-orthogonal signal and idler photons(emitted from the nonlinear crystal),two sideband HOM-shape dips appear besides the MZ interferometric fringe envelope with a separation interval that is twice that of the group delay difference.By measuring the HOM-shape dip depth,the separation between the two sideband dips as well as the MZ fringed envelope width,both the spectral indistinguishability and the temporal entanglement parameter Rt can be simultaneously quantified.The spectral indistinguishability is VHOM=0.52±0.02 and the degree of frequency entanglement is Rt=15.8±0.8?3.The quantum measurement of Sagnac effect was achieved using a frequency entanglement source.Based on the Sagnac effect,the conveyor-belt quantum time syn-chronization protocol is theoretically extended.The same Sagnac loop A and Sagnac loop B are introduced respectively in the ground A and ground B.Starting from the protocol,the same acceleration is introduced to the Sagnac loop A and the Sagnac loop B,and the starting time of the uniform acceleration of Sagnac loop A and loop B cor-responds to the start time of clock A and clock B respectively.At time t,the clock difference is related to the delay difference between Sagnac loop A and Sagnac loop B.By measuring the offset of HOM dip position,the clock difference can be recovered.We built this protocol experimental system and evaluated the stability of the system with fs magnitude,that is,this protocol has the fs magnitude of time synchronization stability.4.We propose a slope locking method to optimize the HOM interference locking loop in quantum time synchronization systems.The faster feedback rate,higher locking SNR and higher stability of the locking loop are realized.At the same time,it is found that the locking stability of the HOM interference loop is still fs magnitude when the coincidence counting rate of entangled two-photon is 70/s,which provides support for quantum time synchronization with long distance and higher accuracy.5.We have studied the two-photon interference phenomenon of multi-level cas-caded HOM interferometer theoretically and experimentally.The two-photon state after the incident cascade 50/50 beam splitter are explained by means of Feynman-like dia-gram.By observing the dip position of the second-order quantum interference pattern,the arm length differences between the cascaded HOM interferometer can be obtained intuitively.The measurement accuracy of subpicosecond level and the measurement stability of femtosecond level are realized experimentally,which is an efficient and simple quantum delay measurement method.