High-performance eight-channel system with fractal superconducting nanowire single-photon detectors

Superconducting nanowire single-photon detectors(SNSPDs)have become a mainstream photon-counting technology that has been widely applied in various scenarios.So far,most multi-channel SNSPD systems,either reported in literature or commercially available,are polarization sensitive,that is,the system detection efficiency(SDE)of each channel is dependent on the state of polarization of the to-be-detected photons.Here,we reported an eight-channel system with fractal SNSPDs working in the wavelength range of 930 to 940 nm,which are all featured with low polarization sensitivity.In a close-cycled Gifford-McMahon cryocooler system with the base temperature of 2.2 K,we installed and compared the performance of two types of devices:(1)SNSPD,composed of a single,continuous nanowire and(2)superconducting nanowire avalanche photodetector(SNAP),composed of 16 cascaded units of two nanowires electrically connected in parallel.The highest SDE among the eight channels reaches 96+^(4)_(-5%),with the polarization sensitivity of 1.02 and a dark-count rate of 13 counts per second.The average SDE for eight channels for all states of polarization is estimated to be 90±5%.It is concluded that both the SNSPDs and the SNAPs can reach saturated,high SDE at the wavelength of interest,and the SNSPDs show lower dark-count(false-count)rates,whereas the SNAPs show better properties in the time domain.With the adoption of this system,we showcased the measurements of the second-order photon-correlation functions of light emission from a singlephoton source based on a semiconductor quantum dot and from a pulsed laser.It is believed that this work will provide new choices of systems with single-photon detectors combining the merits of high SDE,low polarization sensitivity,and low noise that can be tailored for different applications.

Field and current driven versions of Brandt method for calculating transport ac loss of superconducting cylinder and strip

As an elegant and fast numerical tool for solving time‐dependent electromagnetic field problems in hard superconductors,Brandt’s method has played an important role in understading the magnetic behavior of superconducting strips,discs,bars and cylinders in various aspect ratios.However,the application of this convenient method was mainly in magnetization processes.Traditionally,the solution of current transport problem needs to introduce a driving electric field E_(a),which requires a low efficiency iterative process and E_(a) itself was not clearly explained.In this work,three integral algorithms based on the Brandt’s method are developed to deal with current transport problems,which directly adopt the applied current as a boundary condition.Namely the current(I)‐driven version and two current‐field‐driven versions A and B.Moreover,the arbitrary applied magnetic field can also be included in the I‐driven version.The derivation with all necessary formulas for the methods are given in this work.As an example,the new methods,as well as the traditional method are used for calculating transport ac loss Q of a superconducting cylinder or strip obeying a power‐law relation of E∝J^(n)as a function of a given I(t).Derived from the Ampère law and the differential rather than the integral expression of the Faraday law,the current‐driven version can be used for more accurate and much quicker computation.Being an intermediate quantity,E_(a)(t) in the two current‐field‐driven versions is accurately calculated under the given I(t),but version B is much quicker than A.Problems relating to E_(a)(t) and Q stabilization process are discussed.