Highly efficient NbTiN nanostrip single-photon detectors using dielectric multilayer cavities for a 2-µm wavelength band.

We report superconducting nanostrip single-photon detectors (SNSPDs) with dielectric multilayer cavities (DMCs) for a 2-µm wavelength. We designed a DMC composed of periodic SiO2/Si bilayers. Simulation results of finite element analysis showed that the optical absorptance of the NbTiN nanostrips on the DMC exceeded 95% at 2 µm. We fabricated SNSPDs with an active area of 30 µm × 30 µm, which was sufficiently large to couple with a single-mode fiber of 2 µm. The fabricated SNSPDs were evaluated using a sorption-based cryocooler at a controlled temperature. We carefully verified the sensitivity of the power meter and calibrated the optical attenuators to accurately measure the system detection efficiency (SDE) at 2 µm. When the SNSPD was connected to an optical system via a spliced optical fiber, a high SDE of 84.1% was observed at 0.76 K. We also estimated the measurement uncertainty of the SDE as ±5.08% by considering all possible uncertainties in the SDE measurements.

Photon detection at 1 ns time intervals using 16-element SNSPD array with SFQ multiplexer.

We demonstrate the high-speed operation of a 16-element superconducting nanostrip single-photon detector (SNSPD) array with a single flux quantum (SFQ) multiplexer. The SFQ multiplexer can reshape the output signals from 16-element SNSPD into pulses with durations shorter than 1 ns and bundle these pulses into one output line, which is advantageous for high-speed operation of the SNSPD array system. We confirmed the correct operation of the 16-element SNSPD system with a system detection efficiency of 80% at a wavelength of 1550 nm, timing jitter of 45 ps, and successful observation of photons at 1 ns time intervals as distinguishable output pulses. The reduction in detection efficiency could also be suppressed to ∼0.93 during the dead time of ∼10ns for each SNSPD pixel when the incident photon flux was relatively low at 0.1 photon/pulse.

Scalable readout interface for superconducting nanowire single-photon detectors using AQFP and RSFQ logic families.

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.

Scalable implementation of a superconducting nanowire single-photon detector array with a superconducting digital signal processor.

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.

High-time-resolved 64-channel single-flux quantum-based address encoder integrated with a multi-pixel superconducting nanowire single-photon detector.

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.