The rapid development of superconducting nanowire single-photon detectors (SNSPDs) over the past decade has led to numerous advances in quantum information technology. The record for the best system detection efficiency (SDE) at an incident photon wavelength of 1550 nm is 93%. This performance was attained from an SNSPD made of amorphous WSi; such SNSPDs are usually operated at sub-Kelvin temperatures. In this study, we fabricated an SNSPD using polycrystalline NbN. Its SDE is 90.2% at 2.1 K for incident photons with a 1550-nm wavelength, and this temperature is accessible with a compact cryocooler. The SDE saturated at 92.1% when the temperature was lowered to 1.8 K. We expect the results lighten the practical and high performance SNSPD to quantum information and other high-end applications. Main textA single-photon detector with high detection efficiency is the key enabling technology for quantum information and various applications, including the test of loophole-free Bell inequality violation 1 , quantum teleportation 2 , measurement-device-independent quantum key distribution 3 , and linear optical quantum computation 4 . Superconducting single-photon detectors outperform their semiconducting counterparts in terms of not only detection efficiency but also dark count rate, timing jitter, and counting rate 5 . In the case of the telecommunication wavelength (1550 nm), the highest system detection efficiency (SDE) greater than 90% has been reported for two types of detectors. One is a transition edge sensor (TES) made of tungsten (W), with an SDE of 95% 6 ; the other is a superconducting nanowire single-photon detector (SNSPD) made of amorphous WSi, with an SDE of 93% 7 . However, because of the low superconducting transition temperature of W and WSi, the requirement of sub-Kelvin cryogenics represents a burden for practical applications. Many studies focused on SNSPDs fabricated using different materials and aiming to obtain a high SDE at higher operating temperatures have been reported [8][9][10][11] ; however, none of these attempts has been successful. Regarding another important parameter, timing jitter, a WSi SNSPD and a W TES have values of approximately 150 ps and 50-100 ns, respectively, which limits their use in

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Atomic-scale metal films exhibit intriguing size-dependent film stability, electrical conductivity, superconductivity, and chemical reactivity. With advancing methods for preparing ultra-thin and atomically smooth metal films, clear evidences of the quantum size effect have been experimentally collected in the past two decades. However, with the problems of small-area fabrication, film oxidation in air, and highly-sensitive interfaces between the metal, substrate, and capping layer, the uses of the quantized metallic films for further ex-situ investigations and applications have been seriously limited. To this end, we develop a large-area fabrication method for continuous atomic-scale aluminum film. The self-limited oxidation of aluminum protects and quantizes the metallic film and enables ex-situ characterizations and device processing in air. Structure analysis and electrical measurements on the prepared films imply the quantum size effect in the atomic-scale aluminum film. Our work opens the way for further physics studies and device applications using the quantized electronic states in metals.

In this paper, a new formula is proposed to improve the accuracy of spot position measurements on an InGaAs quadrant detector (QD). It is obtained by analyzing the relationship between the light spot position and the output signals of the QD and combining the infinite integral method with the Boltzmann method due to their opposite error characteristics. Based on the proposed formula, the measurement accuracy can be improved greatly, which is confirmed by the simulation and experimental results. In addition, it requires fewer parameters compared with the polynomial method when reaching the same accuracy. Thus, the new formula can be practical in applications of spot position measurements.

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In this paper, ON-OFF Keying free-space optical (FSO) communication with coherent detection and double adaptive detection thresholds is proposed. The 3-bit encoding and double adaptive detection thresholds presented in the scheme can significantly improve the system performance, and there is no need of acquiring the instantaneous channel state information and probability density function (pdf) of the turbulence model. In order to enhance the accuracy of the threshold even further, the decision-aided method is offered to set the second detection threshold. The pdfs of the detection thresholds and the average bit error rate (BER) of the system are theoretically derived. Numerical studies show that the performance of the system is comparable to the performance of the idealized adaptive detection system and the loss of the signal-to-noise ratio (SNR) performance is only 1 dB at a BER of 10 −9 over a lognormal turbulence channel with normalized standard deviation of irradiance σ = 0.25 and phase noise with normalized variance σ 2 φ = 0.07. Hence, this scheme can contribute to the performance improvement of the FSO system and its practical realization.

A packaged high performance ZnMgO solar-blind UV photodetector is prepared via a silica gel sealing treatment. The responsivity and stability of the device is improved according to this sealing treatment.