A two-dimensional single-photon imaging system with high sensitivity and high time resolution is the ultimate camera and useful in a wide range of fields. A superconducting nanowire single-photon detector (SSPD or SNSPD) is one of the best candidates for realizing such an ultimate camera due to its high detection efficiency in a wide spectral range, low dark count rate without after-pulsing, and excellent time resolution. Here we propose a new readout scheme to realize a large-scale imaging array based on SSPD, where a row-column readout architecture is combined with a digital signal processor based on a single-flux-quantum (SFQ) circuit. A 16-pixel row-column readout SSPD array is fabricated and measured with an SFQ digital signal processor. We successfully acquired spatial information as encoded digital bit codes with the temporal information of the photon detection. The system timing jitter was measured as <80 ps for all 16 pixels even through the SFQ signal processor, indicating the potential for an imaging array with an extremely high time resolution.
A two-dimensional single-photon imaging system with high sensitivity and high time resolution is the ultimate camera and useful in a wide range of fields. A superconducting nanowire single-photon detector (SSPD or SNSPD) is one of the best candidates for realizing such an ultimate camera due to its high detection efficiency in a wide spectral range, low dark count rate without after-pulsing, and excellent time resolution. Here we propose a new readout scheme to realize a large-scale imaging array based on SSPD, where a row-column readout architecture is combined with a digital signal processor based on a single-flux-quantum (SFQ) circuit. A 16-pixel row-column readout SSPD array is fabricated and measured with an SFQ digital signal processor. We successfully acquired spatial information as encoded digital bit codes with the temporal information of the photon detection. The system timing jitter was measured as <80 ps for all 16 pixels even through the SFQ signal processor, indicating the potential for an imaging array with an extremely high time resolution.
We propose a single-flux-quantum (SFQ) based event-driven encoder for a large-pixel superconducting nanowire single-photon detector (SSPD) array with a row-column readout architecture. The designed SFQ encoder can acquire time-tagged address data with a high time resolution. To apply 32 × 32 pixel SSPD system with a row-column architecture, the SFQ encoder is equipped with two 32-channel input ports among which one is for positive pulses, whereas the other is for negative pulses. The number of Josephson junctions (JJs) in the designed circuit is 2780. The power dissipation and required total bias current are 0.21 mW and 170 mA, respectively, owing to the reductions in critical currents of the JJs and bias resistors. The circuit generates time-tagged address data by applying electrical pulses at room temperature not only in liquid helium but also in a 0.1-W Gifford-McMahon (GM) cryocooler.
We propose a scalable readout interface for superconducting nanowire single-photon detector (SSPD) arrays, which we call the AQFP/RSFQ interface. This interface is composed of adiabatic quantum-flux-parametron (AQFP) and rapid single-flux-quantum (RSFQ) logic families. The AQFP part reads out the spatial information of an SSPD array via a single cable, and the RSFQ part reads out the temporal information via a single cable. The hybrid interface has high temporal resolution owing to low timing jitter in the operation of the RSFQ part. In addition, the hybrid interface achieves high circuit scalability because of low supply current in the operation of the AQFP part. Therefore, the hybrid interface is suitable for handling many-pixel SSPD arrays. We demonstrate a four-pixel SSPD array using the hybrid interface as proof of concept. The measurement results show that the hybrid interface can read out all of the pixels with a low error rate and low timing jitter.
Detecting spatial and temporal information of individual photons is a crucial technology in today's quantum information science. Among the existing single-photon detectors, superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated with a sub-50 ps timing jitter, near unity detection efficiency1, wide response spectrum from visible to infrared and ~10 ns reset time. However, to gain spatial sensitivity, multiple SNSPDs have to be integrated into an array, whose spatial and temporal resolutions are limited by the multiplexing circuit. Here, we add spatial sensitivity to a single nanowire while preserving the temporal resolution from an SNSPD, thereby turning an SNSPD into a superconducting nanowire single-photon imager (SNSPI). To achieve an SNSPI, we modify a nanowire's electrical behavior from a lumped inductor to a transmission line, where the signal velocity is slowed down to 0.02c (where c is the speed of light). Consequently, we are able to simultaneously read out the landing locations and arrival times of the photons from the output electrical pulses using only two connections. We have demonstrated single-photon imaging by using a 19.7 mm long SNSPI, which is meandered into an imaging area of 286 {\mu}m*193 {\mu}m. The nanowire has a temporal resolution of 50 ps for detecting 1.5 {\mu}m photons. The 2D spatial resolution is 13.0 {\mu}m in the vertical direction and 5.6 {\mu}m in the horizontal direction. The maximum number of resolvable locations (i.e., the effective number of pixels) in such a long nanowire is calculated to be 590. Rather than operating an individual detector in a scanning mode or using SPD arrays, this SNSPI gives an alternative approach to taking large-scale single-photon images and measuring temporal and spatial correlation.
We report a 16-pixel (4×4) superconducting nanowire single-photon detector (SNSPD) array featuring pixel-level photon-number-resolving capability. This multidimensional sensing is enabled by integrating a compact four-element series nanowire architecture into each spatial pixel, allowing direct discrimination of one to four photons. Under single-mode fiber illumination, the device achieves a total system detection efficiency of 41.5%, given the array geometrical fill factor of 53%. Across all channels, the device exhibits reliable multiphoton discrimination, maintaining minimum readout fidelities of 96%, 89%, 73%, and 77% for one- to four-photon states, respectively. Furthermore, independent channel evaluations reveal low timing jitter and negligible inter-channel crosstalk (<0.1 % ). By implementing differentiating pulse shaping for high-flux operation, a maximum single-channel −3 dB count rate of 984 MHz is achieved, establishing an aggregated array bandwidth potential of 8.9 GHz. This sensing architecture may provide a viable pathway toward high-flux single-photon imaging, offering potential applications in quantum optics and light detection and ranging (LiDAR) systems.
We developed a monolithic integration process for a superconducting nanostrip single-photon detector (SNSPD) and a cryogenic signal processor based on a single-flux-quantum (SFQ) circuit. Sixteen-pixel SNSPDs consisting of a 10-nm-thick and 100-nm-wide NbTiN nanowire were fabricated on a 3-in. Si wafer with an SFQ merger fabricated by the National Institute of Advanced Industrial Science and Technology Nb 2.5 kA/cm2 standard process 2. Subsequently, we illuminated the photons on the 16-pixel SNSPD array through an optical fiber and obtained a similar bias-current dependence of the detection efficiency with a single-pixel SNSPD without the SFQ merger. This indicates that the SFQ merger works correctly, and that the power dissipation of the SFQ circuit, which is estimated to be 200 μW, does not deteriorate the photon detection of SNSPD fabricated on the same chip with a size of 5 × 5 mm2. Furthermore, we measured timing jitter using a pulsed laser as the photon source. Owing to the careful tuning of the wiring length between each pixel of the 16-pixel SNSPD and each input port of the SFQ merger, the obtained timing jitter was 41.4 ps, which is approximately equal to or less than that of the single-pixel SNSPD.
We propose and demonstrate a 64-channel event-driven encoder based on single-flux quantum (SFQ) circuits for application to a multi-pixel superconducting nanowire single-photon detector (SSPD) system. Multi-pixel SSPD systems were developed to improve maximum count rates and realize ultra-sensitive imaging systems. An intelligent signal processor is required, which we designed based on SFQ circuits. We were able to obtain an address based on the reserve timing information for the photon detection that occurs in the SSPD, as well as read the time-tagged address information for the SSPDs through the SFQ encoder. The overall observed FWHM jitter was 56.5 ps.