Research On The Technology Of Photon Coincidence Counting And Laser Phase Locking

Quantum computation constitutes a central topic attracting the focus of researchers,which promises to solve classically difficult problems and may bring about a industrial revolution.Nowadays,researchers have performed lots of experiments towards the ver-ification of the quantum computers in different experimental platforms,such as linear optics,trapped ions,neutral atoms and nitrogen-vacancy center.Because of the long life-time,high controllability and detectability of photons,linear optics plays an im-portant role in the field of quantum information under the rapid development of the corresponding photon source and photon manipulation techniques,upon which high expectation is placed for approaching the quantum computers.As the number of parallel photons and detection channels increases rapidly,con-tinuous improvement of the coincidence counting techniques is in need for exploring the multi-photon quantum state accordingly.On the other hand,a stable quantum en-tangled state can be built on the interference between photons in different wavelength or through complex optics,which requires the stabilization of the relative laser phase.So it is important to develop efficient and robust phase lock techniques.Focusing on the requirements of photonic quantum computing experiments,this thesis illustrates the design and implement of one multi-channel photon coincidence counting platform.This platform is excellently applied in the 12-photon entanglement experiment and the 30-photon boson sampling experiment.Meanwhile,the thesis in-vestigates the laser phase-locking problem for the photonic interference.The design and development of the techniques to stabilize the relative phase of two independent lasers is explained,which promises to provide the stable entangled state phase for cold-atom quantum memory experiments.First,currently in both the 12-photon entanglement experiment and the 30-photon boson sampling experiment,researchers are able to control tens of photons,demanding the multi-channel photon coincidence counting measurements.For probing large-scale quantum states,the number of detection channels is improved with the application of the backplane-board structure.A coincidence counting system containing up to 104 input channels to receive signals from different single photon detectors is developed.Mean-while,the automatic precise alignment of multi-channel pulses is realized by combining the rough and fine delay adjusting operations,which is a much more time-saving way than the conventional alignment method to modify cables’ length manually.The width of coincidence windows can be freely changed in the order of several nanoseconds.Moreover,in each input channel,a digital discriminator is exploited to achieve the flex-ible and adjustable screen domain,and consequently the output signals of single photon detectors ranging from 0.01 V to 5 V can be identified and addressed.Secondly,in the two cold-atom entangled ensemble experiment,two independent lasers in slightly different wavelengths should have a locked relative phase for gener-ating a stable interference.For constructing the locking system,a high-speed analog-digital convertor is used to collect the interference strength in real-time.Afterwards,the relative phase is extracted in the FPGA components,based on which a subsequent phase compensation is made by driving a electro optical modulator.In present,the basic phase-locking function has been verified in optical and electric tests.The application in the experiment leads to a phase noise of 0.0949rad,which requires further optimization and improvement.Finally,photon-number-resolved single photon detectors can distinguish multiple photons reaching the same input channel,which can effectively facilitate the large-scale boson sampling experiments.For addressing the low-amplitude small-width output pulses of these detectors,the pulse broadening and amplifying circuits are designed,matching the input of the high-resolution high-speed ADC.As a result,the read-out circuit with the capability to resolve the signals with amplitudes higher than 0.5 mV,which lays a basis for implementing the photon-number-resolved coincidence counting measurement.The innovations of this thesis can be listed as follows:(1)The problem to make coincidence counting measurements for over one-hundred input channels is solved.Besides,the functions involving inter-channel pulse auto-matic alignment,the inter-channel coincidence counting,the photon-number-resolved coincidence counting and the coincidence-based real-time control&feedback have been realized,which successfully assists the ten-photon entanglement,the twelve-photon en-tanglement and the photonic quantum repeater experiments.Currently,the coincidence counting platform provides the key technical support in the experiments of photonic quantum supremacy and two-Rydberg-atom entanglement.(2)A JTAG-based modular instrument management bus is proposed and realized.The automatic check module is designed on the basis of the daisy chains,which avoids the probability of broken chains.The functions of the firmware download,the boundary scan and the online debug are supported.Besides,this bus occupies a small quantity of pins and can be compatible with different soft-and hardware.(3)The relative phase between independent lasers is extracted by measuring the laser interference.The phase feedback and compensation with the phase folding is ver-ified,which can be used to implement the phase locking between two lasers in different wavelengths.