Study On Spectral Characteristics Of Quantum Light Source In Near-infrared And Mid-infrared Bands

Quantum light sources include single-photon sources and entangled light sources,which are the carriers of information in quantum information processing applications,and also important fundamental resources in quantum information research.According to the operating wavelengths,quantum light sources can mainly be classified into visiblewavelengths,near-infrared(NIR)-wavelengths,mid-infrared(MIR)-wavelengths,and other-wavelengths quantum light sources.Currently,commonly used quantum light sources are in the NIR band,such as the 810 nm and 1550 nm bands,while research on quantum light sources in the MIR band(about 2-20 μm)is in its infancy.Although many advances have been made,there is still a lack of high-quality quantum light sources in the near-and mid-infrared wavelengths.How to prepare high-brightness,spectral-uncorrelated and easy-to-operate quantum light sources in the NIR and MIR bands is still a challenging problem.To address this problem,we explore the quantum light sources in NIR and MIR bands in this thesis,mainly including the following two works.(1)In the 810 nm,Hong-Ou-Mandel(HOM)interference based on tightly focused pumping to generate entangled photons is studied.In this work,the biphoton spectral distribution and HOM interference generated by the BBO crystal under the condition of tight focusing are systematically studied for the first time.Theoretical calculations show that using a lens with a 50-mm focusing length,the spectral width of the down-converted photons is increased by 7.9 times that of the non-focused case;the width of the HOM interference fringe is reduced to 1/8,and the visibility of the interference fringe increases from 53.0% to 98.7%.We experimentally prepare the energy-time entanglement state by using type-II BBO crystal and perform HOM interference,thereby obtaining the interference visibility of(86.6 ± 1.0)%.The increase in the HOM visibility is due to the improvement of biphoton’s spectral symmetry.(2)In the 3.0 μm,the Optimization of periodically polarized lithium niobate(PPLN)crystals to generate frequency-domain pure-state single-photon sources is based on a particle swarm optimization algorithm.In this work,we present a method for designing lithium niobate(LN)crystal using a metaheuristic algorithm.Specifically,we use a particle swarm optimization algorithm to optimize the duty cycle of the PPLN crystal polarization cycle.With our optimization,the spectral purity can be improved from 0.820 to 0.998 under the third group-velocity-matched(GVM)condition,and the wavelength-tunable range is from 3.0 μm to 4.0 μm for the degenerated case and 3.0 μm to 3.7 μm for the nondegenerated case.This work is important for the development of quantum sensing,quantum imaging,and quantum communication in the mid-infrared band.