The Superconducting Nano Strip Photon Detector (SNSPD) is a single photon detector that uses a superconducting nanowire with a width of ~100 nm and a thickness of ~5 nm. It was first developed by Golt'sman et al. in 2001. [1] SNSPD shows extremely high performance in detection efficiency, dark count rate, operating speed, time jitter, etc., and has been greatly contributed to the progress of research in various advanced fields that were previously impossible due to the limitations of photon detectors. Currently, research and development of SNSPDs are being actively carried out with the aim of further improving performance, improving operating temperature, and increasing the number of devices (single-photon camera). [2]
SNSPD is sometimes called a superconducting nanowire single photon detector (SNSPD) or a superconducting single photon detector (SSPD), however, the international standard organization IEC/TC90 defines it as a Superconducting (Nano) Strip Photon Detector. [3] This is because the width of the superconducting wire is more than 10 times its thickness, and it is a two-dimensional tape (strip) rather than a one-dimensional nanowire.
The main results of SNSPD development in our laboratory are introduced below.

・SNSPD with ultralow dark-count rate

SNSPD is an ultra-high-sensitivity broadband photodetector capable of single-photon detection over a wide wavelength range from ultraviolet to infrared. Let us consider the case of inputting single-photons from a weak laser light source with a wavelength of 1.5μm through an optical fiber. As shown in the figure on the left, in addition to single-photons at 1.5μm, SNSPD also detects extremely weak broadband light from external light source (orange in the figure on the left), which is transmitted through the coating of the optical fiber. This broadband light is the dominant noise of SNSPD. The noise can be reduced by introducing an optical filter that only transmits light with a wavelength of 1.5μm, as shown in the middle of the figure, and blocking broadband light from the external light source. However, room-temperature blackbody radiation (yellow-green in the center of the figure) emitted by the optical filter itself becomes a new noise source. Our laboratory succeeded for the first time in significantly reducing the dark count rate of the SNSPD to less than 1/10000 by cooling the optical filter and suppressing the black body radiation emitted from the optical filter. In addition, using the developed SNSPD with ultralow dark count, we realized quantum cryptography communication of 340 km, which was the world record at that time. [4]

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・SNSPD using MgB2

Usually, the working temperature of SNSPD is below 3K, since niobium nitride (NbN) with Tc=16K or tungsten silicide (WSi) with Tc=5K is used as superconducting material for SNSPD. Our laboratory succeeded in developing SNSPD working at 11K using magnesium diboride (MgB2) with Tc=39K for the first time. The SEM image (76 nm line width) of the fabricated device is shown on the left, and a schematic diagram and photograph of the MBE equipment for MgB2 thin film growth are shown in the center of the figure. We are currently conducting research to improve the detection efficiency of MgB2-SNSPD. [5]

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・SNSPD using MoN

Our laboratory has developed an SNSPD using molybdenum nitride (MoN). MoN has a slightly lower Tc than NbN, but longer electron-phonon relaxation time, which is effective for high detection efficieny. By simulating the absorption rate using the FDTD method assuming a cavity structure as shown on the left side of the figure, very high absorption rate of the MoN nanowire is expected. Microscopic and SEM images (size: 15μm square, line width: 100 nm) of the device fabricated based on the optimized design are shown in the center of the figure, and the dependence of the detection efficiency on the bias current is shown on the right. The detection efficiency saturates over a wide range of bias currents, indicating that MoN is a promising material for SNSPDs. [6]

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・SNSPD on Si waveguide

When applying SNSPDs to optical quantum information processing such as optical quantum computers, SNSPDs fabricated on optical waveguides, which can be used as part of optical integrated circuits similar to semiconductor integrated circuits, are required. We developed a Si waveguide-coupled SNSPD in collabration with NTT Laboratories. A schematic diagram of the fabricated device is shown on the left side of the figure. The coupling loss of the spot-size converter is as low as 1.9dB at low temperature (3K), resulting in an on-chip detection efficiency of 94% and a system detection efficiency of 40%, both significantly better than previously reported (center of the figure). Furthermore, by introducing a cooled arrayed waveguide grating (AWG) as shown in the right figure, we successfully reduced the dark count rate by two orders of magnitude. [7]

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・SNSPD with μm-width (SMSPD)

SNSPDs usually use superconducting nanowires with a line width of about 100 nm. Recently, it has been reported that single-photon detection is possible even when the linewidth is μm, when the bias current near to the depairing current is applied. We fabricated a NbTiN single thin wire with a width of 1μm and a length of 40 μm as shown in the left figure, and evaluated its characteristics when the load resistance (Rsh) was changed. As a result, it is possible to detect a single photon even with a thin wire of μm width. On the other hand, it was found that the time jitter also increased (right figure). [8]

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・SNSPD using cuprate superconductors

Cuprate superconductors are superconducting materials with high Tc up to 135K and are called high temperature superconductors. SNSPDs using cuprate superconductors are expected to operate at high temperatures and at high speeds. We have grown an ultra-thin La1.85Sr0.15CuO4 film with Tc=42K as shown in the left figure, and fabricated nanowires with a width of 100 nm, a thickness of 5 nm, and a length of 10 μm (center of the figure). The right side of the figure shows the current-voltage characteristics and photoresponse of nanowires. In the IV characteristics, a clear voltage jump and hysteresis were observed in Ic, which is necessary for single-photon detection. When a bias was applied, a voltage pulse corresponding to the optical pulse was observed up to 30K for the first time. [9]

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・SNSPD without polarization dependence

Since SNSPD has a meandering shape in which nanowires are arranged in parallel, the absorption rate becomes large when the electric field vector E of light is parallel to the nanowires, but the absorption rate becomes small when E is perpendicular to the nanowires (left figure). For this reason, the detection efficiency of SNSPDs is polarization dependent, but optical fiber transmission requires polarization-independent optical devices, and various methods have been proposed to eliminate polarization dependence. We have proposed and fabricated a bilayer structure in which two meandering SNSPDs overlap each other so that the nanowires are perpendicular to each other for the first time (center of the figure). The figure on the right shows the polarization dependence of the fabricated two-layer SNSPD. When the direction of the electric field E is changed by rotating the polarizer, the detected number of upper SNSPD and the detected number of lower SNSPD oscillate. However, the sum of the detected number becomes almost unchanged, realizing a polarization-independent SNSPD (polarization dependence C=0.0006). [10]

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[1] G.N. Gol'tsman et al.,"Picosecond superconducting single-photon optical detector," Appl. Phys. Lett.79, 705 (2001).

[2] I. E. Zadeh et al.,"Superconducting nanowire single-photon detectors: A perspective on evolution, state-of-the-art, future developments, and applications," Appl. Phys. Lett.118, 190502 (2021). (download free)
I. Holzman and Y. Ivry,"Superconducting Nanowires for Single-Photon Detection: Progress, Challenges, and Opportunities," Adv. Quantum Technol.2, 1800058 (2019).

[3] IEC 61788-22-3 ED1.

[4] H. Shibata et al.,"SNSPD with ultimate low system dark count rate using various cold filters," IEEE Trans. Appl. Supercond. 27, 2200504 (2017).
H. Shibata et al.,"Ultimate low system dark-count rate for superconducting nanowire single-photon detector," Optics Letters 40, 3428 (2015). (download free)
H. Shibata et al.,"Quantum key distribution over a 72 dB channel loss using ultralow dark count superconducting single-photon detectors," Optics Letters 39, 5078 (2014). (download free)
H. Shibata et al.,"Superconducting nanowire single-photon detector with ultralow dark count rate using cold optical filters," Appl. Phys. Express 6, 072801 (2013).

[5] H. Shibata,"Review of Superconducting Nanostrip Photon Detectors using Various Superconductors," IEICE TRANS. ELECTRON. E104-C, 429 (2021). (download free)
H. Shibata et al.,"Fabrication of MgB2 nanowire single-photon detector using Br2-N2 dry etching," Appl. Phys. Express 7, 103101 (2014).
H. Shibata et al.,"Single-photon detection using magnesium diboride superconducting nanowires," Appl. Phys. Lett. 97, 212504 (2010).

[6] M. Nishikawa et al.,"Fabrication of Superconducting Nanowire Single-Photon detectors using MoN," IEEE Trans. Appl. Supercond.32, 2200104 (2022).

[7] K. Ono et al.,"Si waveguide-integrated superconducting nanowire single photon detectors with arrayed waveguide grating," Proc. SPIE 11806, 118060S (2021)
H. Shibata et al.,"Waveguide-integrated SNSPD with sopt-size converter on Si photonics platform," Supercond. Sci. Technol. 32, 034001 (2019).

[8] K. Ono et al.,"Single photon detection using superconducting micrometer strip" The 54th Japan Society of Applied Physics Hokkaido Branch Meeting, B-12 (2019).

[9] H. Shibata et al.,"Photoresponse of La1.85Sr0.15CuO4 nanostrip," Supercond. Sci. Technol. 30, 074001 (2017).

[10] N. Kaina et al.,"Fabrication of NbN superconducting single-photon detector with minimized polarization dependence," IEICE Technical Report, SCE2010-36 (2011).
H. Shibata et al.,"NbN superconducting single-photon detector with bilayer structure," Physics Procedia 36, 324 (2012).(download free)