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conducting state is locally destroyed is generated. If su-cient bias current is supplied to this superconducting nanowire, the superconducting state of the whole cross-section of the nanowire collapses because the supercon-ducting current density around the hot spot exceeds the critical value (the value over which the superconducting state collapses) by the trigger of hot spot generation. As a result, the resistance between both sides of the nanowire increases to several kΩ and the bias current ows through the load of 50 Ω. en, Joule heat around the hot spot diuses over the substrate and the hot spot area returns to the superconducting state. Finally, the area recovers to the initial state where bias current ows through the supercon-ducting nanowire. When the superconducting nanowire absorbs a photon, a spike-like pulse is observed by moni-toring the voltage of both sides of the nanowire. Hence, a photon can be detected by monitoring this pulse by mea-surement apparatus at room temperature.2.2Development of high detection efficiency SSPD systemAlthough the theory of operation of SSPD is quite simple, there are some technical barriers to overcome to achieve high detection eciency. ere are three factors that determine the detection eciency of SSPD. ey are coupling eciency with optical ber, optical absorptance of nanowire, and pulse generation probability (Fig. 2). We developed a special ber package so that the light from single mode (SM) ber used for the communication wave-length band irradiates on the entire photosensitive area without loss. e photosensitive area of the SSPD is 15 × 15 μm2 which is larger than the diameter of core of the SM ber (about 10 μm∅). We succeeded in achieving ber coupling eciency of almost 100% by fusing a Graded Index (GRIN) lens at the terminal of the ber to focus on the sensitive area [11].e thickness of the superconducting nanowire mem-brane is about 5 nm, which is relatively thin, so it is dif-cult to realize high optical absorptance due to transmission and reection of light in a single layer membrane. Hence, we adopted a device structure called a double side cavity in order to optimize the device structure so that the pho-toelectric eld intensity become maximum near the nanowire by enclosing a photon in between the silicon substrate and metal reection layer. As a result, optical absorptance over 90% for light of wavelength is 1.550 nm was realized. Normally, the ratio of the area of the super-conducting nanowire to the entire detector (lling factor) FiF2　Three major factors determining detection efficiency of SSPDCoupling efficiencyOpticalabsorptionPulse generation probabilityxx3 mm3 mm15 m15 m~ 100%100 nmUltrathin NbN film of ~ 5 nmEB Lithography>90%>90%Double-side cavity structureSample block for fiber couplingFiF1(a) Device structure of SSPD, (b) Photon detection mechanism(a)(b)Superconducting nanowireFilm thickness: ~ 5 nmLine-width: ~ 100 nmSingle Photon↑Bias current(i)Photon absorption(ii)Hotspot creation(iii)Growth of hotspot(iv)Creation of resistive partThermal relaxation4 Quantum Node Technology58　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)