Zhang Wei-jun

NbN superconducting nanowire single photon detector with efficiency over 90% at 1550 nm wavelength operational at compact cryocooler temperature

The rapid development of superconducting nanowire single-photon detectors over the past decade has led to numerous advances in quantum information technology. The record for the best system detection efficiency at an incident photon wavelength of 1550 nm is 93%. This performance was attained from a superconducting nanowire single-photon detector made of amorphous WSi; such detectors are usually operated at sub-Kelvin temperatures. In this study, we first demonstrate superconducting nanowire single-photon detectors made of polycrystalline NbN with system detection efficiency of 90.2% for 1550-nm-wavelength photons at 2.1 K, accessible with a compact cryocooler. The system detection efficiency saturated at 92.1% when the temperature was lowered to 1.8 K. We expect the results lighten the practical and high performance superconducting nanowire single-photon detectors to quantum information and other high-end applications.

Highly Sensitive Tunable Diode Laser Absorption Spectroscopy of CO2 Around 1.31 μm

Fifteen new absorption lines were observed when studying CO_2 absorption spectroscopy by wavelength modulation(WM) technique with a DFB laser.The line intensity we can detect is 2.251 63×10~(-27) cm~(-1)·(moleculecm~(-2))~(-1) at 6.67×10~2 Pa pressure,corresponding to an absorbance of 3.88×10~(-8).The overtone spectra of CO_2 around 1.31 μm have been measured with a tunable diode laser and the corresponding spectral parameters(positions,intensities,and self-broadening coefficients) are presented.

New Travelling Wave Solutions to Compound KdV–Burgers Equation*

The compound KdV–Burgers equation and combined KdV–mKdV equation are real physical models concerning many branches in physics. In this paper, applying the improved trigonometric function method to these equations, rich explicit and exact travelling wave solutions, which contain solitary-wave solutions, periodic solutions, and combined formal solitary-wave solutions, are obtained.

Absorption measurements for highly sensitive diode laser of CO2 near 1.3μm at room temperature

Fifteen new absorption lines were observed when studying CO2 absorption spectroscopy by wavelength modulation technique with a distributed feedback laser. The overtone spectra of CO2 around 1.31μm and the corresponding spectral parameters (i.e. positions, intensities, self-broadening coefficients) are presented. The intensity of the weakest line detected is 2.25163×10−27cm−1/(molecule.cm−2) at the pressure of 667Pa, with a corresponding absorption of 3.88×10−8.

Interstitial vortex in superconducting film with periodic hole arrays

The response of superconducting Nb films with a diluted triangular and square array of holes to a perpendicular magnetic field are investigated. Due to small edge-to-edge separation of the holes, the patterned films are similar to multi-connected superconducting islands. Two regions in the magnetoresistance R(H) curves can be identified according to the field intervals of the resistance minima. Moreover, in between these two regions, variation of the minima spacing was observed. Our results provide strong evidence of the coexistence of interstitial vortices in the islands and fluxoids in the holes.

Matching effect in superconducting NbN thin film with a square lattice of holes

A square hole array is fabricated over a micro-bridge of NbN thin film by electron beam lithography and reactive ion milling. Magneto-resistance is measured across the micro-bridge filled with a hole array near the superconducting transition temperature. It is found that magneto-resistance minima occur when the number of vortices is an integer multiple or a fractional multiple of the number of holes. The temperature and the current dependences of the matching effect are studied.

Temperature-dependent far-infrared properties of Bi12GeO20 single crystal

Far-infrared reflectivity spectra of bismuth germanium oxide (Bi12GeO20) single crystals are measured from room temperature down to 10 K. All the reflectivity spectra are fitted to a complex dielectric function (ω) in the factorised form. Phonon modes at low frequency are found to develop upon decreasing temperature and gain considerable spectrum weight below 150 K. The temperature-dependent static dielectric constants are obtained from the Lyddane–Sachs–Teller relation based on the obtained oscillator parameters. The dielectric constants are found to increase upon decreasing temperature, which is attributed to the charge transfer among the ions in the unit cell with the temperature varying.

Statistical analysis of the temporal single-photon response of superconducting nanowire single photon detection*

A new method to study the transient detection efficiency (DE) and pulse amplitude of superconducting nanowire single photon detectors (SNSPD) during the current recovery process is proposed — statistically analyzing the single photon response under photon illumination with a high repetition rate. The transient DE results match well with the DEs deduced from the static current dependence of DE combined with the waveform of a single-photon detection event. This proves that static measurement results can be used to analyze the transient current recovery process after a detection event. The results are relevant for understanding the current recovery process of SNSPDs after a detection event and for determining the counting rate of SNSPDs.Counting rate is a key parameter of superconducting nanowire single photon detectors (SNSPD) and is determined by the current recovery time of an SNSPD after a detection event. We propose a new method to study the transient detection efficiency (DE) and pulse amplitude during the current recovery process by statistically analyzing the single photon response of an SNSPD under photon illumination with a high repetition rate. The transient DE results match well with the DEs deduced from the static current dependence of DE combined with the waveform of a single-photon detection event. This proves that the static measurement results can be used to analyze the transient current recovery process after a detection event. The results are relevant for understanding the current recovery process of SNSPDs after a detection event and for determining the counting rate of SNSPDs.

Energy-time Entanglement Generation in Optical Fibers under Continuous Wave Pumping

Energy-time entangled photon-pairs at telecom band are generated via spon- taneous four wave mixing (SFWM) process in optical fibers under continuous wave (CW) pumping. A time domain filter is used to suppress the impact of noise photons and improve the noise property. The characterization of energy-time entanglement is presented through an experiment of Franson-type interference and a violation of CHSH-Bell inequality. It shows that the SFWM in optical fibers under CW pumping provides a simple and practical way to generate high-quality energy-time entangled photon-pairs at telecom band.

Low temperature laser absorption spectra of methane in the near-infrared at 1.65 μm for lower state energy determination

Direct absorption spectra of the 2ν3 band of methane (CH4) from 6038 to 6050 cm−1 were studied at different low temperatures using a newly developed cryogenic cell in combination with a distributed feedback (DFB) diode laser. The cryogenic cell can operate at any stabilized temperature ranging from room temperature down to 100 K with temperature fluctuation less than ±1 K within 1 hour. In the present work, the CH4 spectra in the range of 6038–6050 cm−1 were recorded at 296, 266, 248, 223, 198, and 176 K. The lower state energy E″ and the rotational assignment of the angular momentum J were determined by a “2-low-temperature spectra method using the spectra recorded at 198 and 176 K. The results were compared with the data from the GOSAT and the recently reported results from Campargue and co-workers using two spectra measured at room temperature and 81 K. We demonstrated that the use of a 2-low-temperature spectra method permits one to complete the E″ and J values missed in the previous studies.