HTML5 Webook

65/84

present SSPD is several tens of MHz. is value is not advantageous comparing with other competing techniques such as APD, etc. Also, in some application cases (espe-cially used in visible wavelength), it is necessary to couple it with multimode ber of core diameter 50 μm that is larger than SM ber, so a larger sensitive area is necessary. As a result, the dead time increases and maximum count rate decreases.In order to overcome the limit of maximum count rate by LK, a multi-pixel SSPD is proposed [16]. e merits of multi-pixel are summarized in Fig. 6. It is possible to shorten the dead time without deteriorating the detection eciency by downsizing each pixel and keeping the sensi-tive area necessary for coupling with ber by multi-pixel. Multi-pixel is eective to suppress increase of dead time due to large area.Also, a single pixel SSPD cannot count incident pho-tons, but a multi-pixel SSPD can detect multiple photons incident into individual pixels simultaneously, so photon counting is possible in a pseudo way.Multi-pixel of a million-pixel class would enable real-ization of an ultimate camera with photon-counting-level sensitivity in the future. e most severe bottleneck to realize a multi-pixel SSPD is read-out of the output signal. In general, broader-band coaxial cable is a better thermal conductor. Hence, the number of cables installed in a small mechanical cryocooler is limited due to thermal load to the cryocooler. In order to reduce the number of readout cables, NICT proposed cryogenic signal processing consists of a single ux quantum (Single Flux Quantum: SFQ) logic circuit for the rst time in the world [17]. We have succeeded in signal readout of SSPD and multiplexing [18][19], cross-talk free operation of an SFQ circuit of a 4-pixel SSPD covering signal multiplex [20], demonstration of signal readout with lower timing jitter compared with the conventional method using readout circuit [21]. Also, as a larger-scale multi-pixel device, we are developing a 64-pixel SSPD imaging system. We have already evaluated the individual detection eciency of each pixel of the SSPD and succeeded in reproducing a beam prole of ber ir-radiation [22]. Now, we are developing an encoder circuit for a 64-pixel SSPD. e encoder circuit is a circuit to read out coded position information of a pixel that detected a photon by one coaxial cable. is enables real-time imaging of a 64-pixel SSPD. It is designed so that not only position information of a photon but also time information can be detected by applying an event driven circuit that generates a clock in the circuit every time it detects a photon. Flying time can be measured by measuring the round-trip time of a photon from a certain measuring point and then the distance to the target can be calculated. It is information on the depth of the target, so a three-dimensional image can be constructed using the information on the depth direction in addition to a two-dimensional image recon-structed from the information of a pixel that detected a photon.One of the features of an SFQ circuit is that it operates with low power consumption and the impedance of the circuit is very low. A bias current of about 1 A is necessary to drive a circuit containing ten thousand Josephson junc-tion. Figure 7 shows a microscopic photo of a 64-bit type event driven SFQ encoder circuit. For the preliminary circuit design, bias current of 370 mA was needed to drive the circuit. We tested the circuit by installing it in a cryo-cooler and found temperature increase of the cryocooler due to Joule heat generated in a bias cable [23]. en, we revised the circuit design and succeeded in operating with a small cryocooler by reducing bias current to 150 mA. is 64-bit encoder circuit was installed to the same sample block as the 64-pixel SSPD. We observed output of the SFQ encoder by illuminating light on the SSPD. As a result, information on the address of the pixel that de-tected a photon (binary code) was output synchronously FiF6　Benefit of multi-pixel SSPDSingle pixelMulti-pixelNanowire length：~ 1 mmKinetic inductance LK~ 500 nHDead time：LK/50 Ω~ 10 nsCount rate：~ 20 MHzReduce kinetic inductance・Higher speed and larger area・Pseudo photon-number resolution・Space resolution imaging15 µmFiF7Photograph of 64-bit event-driven SFQ encoder chip and it output waveformtrigger signal from oscillator10110001valid bit= 2+8+32 = 42SFQ outputSpatial informationTiming information614-2 Development of Superconducting Nanowire Single-Photon Detector