Study On Photon Number Resolving SNSPD

In terms of photon-coupled structures,SNSPD can be mainly divided into two types: one is the traditional winding nanowire structure integrated with optical cavity,and the other is the waveguide integrated nanowire structure.Waveguide integrated SNSPD with nanowire structure and photonic number resolution function,is of great significance in the fields of linear quantum optics and quantum computing.Based on the requirements of photonic number resolution function in different technology application fields,the paper studies the photonic number resolution function based on SNSPD with two different optical structures.The main achievements are as follows:1?High quality superconducting thin films are the foundations of devices.We optimized the fabrication of NbN films magnesium oxide,magnesium fluoride and thermal silicon oxide/silicon substrates.By using modern scientific analysis techniques,we characterized microstructure and electrical properties of NbN films.2?On a thermal silicon oxide/silicon substrate,we fabricated an optical cavity SNSPD with a cell nanowire size of 80 nm wide,a gap of 160 nm,and a 4-pixel series Ti resistance in parallel.The experimental results show that at 1550 nm communication band,2.3 K temperature,dark counts of 100 Hz,the detection efficiency of the system reaches 60%,and the resolution capacity is up to 4 photons.3?We designed integrated SNSPD for waveguide with 4-pixel series structure,which constitutes magnesium fluoride material substrates and silicon oxide or silicon nitride material waveguide layers.Besides,we simulated the whole system to achieve an optimization design.The larger refractive index difference between the waveguide material and the substrate can better constrain the photon in the waveguide structure,reduce the probability of photon transmission into the substrate,and thus improve the absorption of the photon by nanowires.When silicon oxide is used as the waveguide material,the waveguide structure is designed to be 3 microns wide,500 nm high and when the nanowire is 60 microns long,the photon absorption rate of the simulation device can reach 98%.When silicon nitride is used as waveguide material,the refractive index difference between the silicon nitride and the magnesium fluoride substrate is larger,and the ability to constrain photons is stronger.The simulated absorptivity of nanowires with the same structure can reach 99%.According to the simulation design results,a 4-pixel series structured nanowire with length of 60 microns,wire width of 60 nm and gap of 100 nm was prepared on the magnesium fluoride substrate.The measurement results show that the dark count corresponding to the offset current is as low as 10 cps when the device count rate is saturated.The time jitter is 58 ps,and the device can realize the resolution of up to 4 photons.