Superconducting Nanowire Single-photon Detector Array Research Articles

Monolithic integrated superconducting nanowire digital encoder

Superconducting digital circuits are promising technologies that can overcome bottlenecks in both classical and quantum computation due to their ultra-high operation speed and extremely low power dissipation. Superconducting nanowire cryotrons (nTrons) are emerging as one type of superconductor switching devices, offering advantages complementary to conventional Josephson junctions. Achieving monolithic integration of a reasonable number of nTrons into a functional digital circuit is a crucial step to extend its application. In this study, we constructed a monolithic integrated nTron-based binary encoder, which includes input fanout circuits, on-chip biasing, combinational logic routing and multi-gate nTrons. This represents a monolithic nTron digital circuit comprising 137 nTron gates, 424 resistors, 274 inductors, and 164 vias developed using a two-superconducting-layer fabrication process. The performance of this monolithic nTron encoder surpasses that of our previously demonstrated circuit with discrete nTron components. The maximum bias margin is 28% for the fanout circuit and 60% for the multi-gate nTron when using a signal generator, while the minimum timing jitter is 40 ps. The total power dissipation mainly from biasing resistors is 19.6 μW, making it more power efficient than RSFQ encoders. The encoder is then packaged and connected with a superconducting nanowire single-photon detector array for demonstrating its function of addressing pixel locations. Compared to the conventional readout, the nTron encoder shows a minimum readout error rate lower than 10−4 and reduces the readout RF lines from 15 to 4. The design and fabrication technologies could enrich integrated nTron digital circuits beyond current limits and promote their applications in classical and quantum systems.

Improving photon number resolvability of a superconducting nanowire single-photon detector array using a level comparator circuit

Photon number resolving (PNR) capability is very important in many optical applications, including quantum information processing, fluorescence detection, and few-photon-level ranging and imaging. Superconducting nanowire single-photon detectors (SNSPDs) with a multipixel interleaved architecture give the array an excellent spatial PNR capability. However, the signal-to-noise ratio (SNR) of the PNR (SNRPNR) of the array will be degraded with increasing the element number due to the electronic noise in the readout circuit, which limits the PNR resolution as well as the maximum PNR number. In this study, a 16-element interleaved SNSPD array was fabricated, and the PNR capability of the array was investigated and analyzed. By introducing a level comparator circuit (LCC), the SNRPNR of the detector array was improved over a factor of four. In addition, we performed a statistical analysis of the photon number on this SNSPD array with LCC, showing that the LCC method effectively enhances the PNR resolution. Besides, the system timing jitter of the detector was reduced from 90 ps to 72 ps due to the improved electrical SNR.

A 64-pixel mid-infrared single-photon imager based on superconducting nanowire detectors.

A large-format mid-infrared single-photon imager with very low dark count rates would enable a broad range of applications in fields like astronomy and chemistry. Superconducting nanowire single-photon detectors (SNSPDs) are a mature photon-counting technology as demonstrated by their figures of merit such as high detection efficiencies and very low dark count rates. However, scaling SNSPDs to large array sizes for mid-infrared applications requires sophisticated readout architectures in addition to superconducting materials development. In this work, an SNSPD array design that combines a thermally coupled row-column multiplexing architecture with a thermally coupled time-of-flight transmission line was developed for mid-infrared applications. The design requires only six cables and can be scaled to larger array sizes. The demonstration of a 64-pixel array shows promising results for wavelengths between 3.4 μm and 10 μm, which will enable the use of this single-photon detector technology for a broad range of new applications.

Approaching pixel-level readout of SNSPD array by inductor-shaping pulse

Although many multiplexed arrays of a superconducting nanowire single-photon detector (SNSPD) have been reported, it is still a major challenge to develop pixel-level readout arrays with high efficiency, parallel detection, and fast processing for real-time imaging. Here, we report a SNSPD array with inductor-shaping pulses for approaching the pixel-level readout. Optimized inductors are introduced to shape the output pulses of each pixel, and the response pulses of all pixels are synthesized in a series-connected structure. Then, the on/off states of all pixels can be encoded to the widths, amplitudes, and areas of the output pulses by the single-channel readout. This proposal is verified by a 4-pixel SNSPD array and a 16-pixel SNSPD array. It shows that the array not only inherits the features of the single-pixel SNSPD, such as photosensitive area, filling factor, quantum efficiency, and dark count rate, but also implements parallel operation of all pixels, which is always confused in traditional multiplexed SNSPD arrays. At the same time, the single-channel readout simplifies the system, and the serial digital signal converted from the shaped pulse enabled an easy and fast readout process, paving the way for high performance and real-time imaging.

Gigahertz Detection Rates and Dynamic Photon-Number Resolution with Superconducting Nanowire Arrays.

Superconducting nanowire single-photon detectors (SNSPDs) have enabled the realization of several quantum optics technologies thanks to their high system detection efficiency (SDE), low dark counts, and fast recovery time. However, the widespread use of linear optical quantum computing, quasi-deterministic single-photon sources, and quantum repeaters requires even faster detectors that can also distinguish between different photon-number states. Here, we present an SNSPD array composed of 14 independent pixels, achieving an SDE of 90% in the telecommunications band. By reading each pixel of the array independently, we show detection of telecommunication photons at 1.5 GHz with 45% absolute SDE. We exploit the dynamic photon-number resolution of the array to demonstrate accurate state reconstruction for a wide range of light inputs, including operation with long-duration light pulses, as obtained with some cavity-based sources. We show two-photon and three-photon fidelities of 74% and 57%, respectively, which represent state-of-the-art results for fiber-coupled SNSPDs.

Cloud height and thickness measurement based on a superconducting nanowire single-photon detector.

At present, the methods for measuring cloud height and thickness mainly include using micro-pulse lidar and microwave radiometer data. To further study cloud height and thickness, a superconducting nanowire single-photon detector (SNSPD) is applied to a lidar system for the first time, to the best of our knowledge, to analyze the cloud height and thickness. In the experiment, a 1.2-m-diameter horizon telescope is used for laser emitting and echo receiving, a 1064nm near-IR pulse laser with a single pulse energy of 4mJ is used as the system emission laser, and a 4-pixel SNSPD array detector is used as the end receiver to complete the echo photon reception. By analyzing the experimental data, the distributions of cloud height and cloud thickness can be obtained using the laser ranging system. The cloud cover condition on a certain day was measured, and the obtained cloud bottom height was about 1222m, cloud top height was about 1394m, and cloud cover thickness was about 172m. The difference between the cloud cover thickness and the forecast value was 28m. The cloud cover height and thickness measured by this method are true and credible.

Improved response model of a superconducting nanowire array for high photon count rate communication

In high-speed photon-counting communication where the slot duration is close to the scale of detector reset time, the detection efficiency recovery characteristics of superconducting nanowire single photon detector (SNSPD) become the key factors affecting counting and communication performance. We introduced the nonlinear detection efficiency recovery model derived from the kinetic inductance model combined with the nonlinear efficiency-current relationship into the pulse position modulation (PPM) optical communication system based on the SNSPD array, and studied the impact of the recovery characteristics on the counting and communication performance of the system. On this basis, we introduce the efficiency recovery model into the correction of the channel logarithmic likelihood ratio (LLR) to estimate the SNR of each slot more accurately. Relevant results show that compared with the existing Poisson channel model, fixed deadtime model and detection efficiency exponential recovery model, this modification combined with FEC can provide the best decoding performance, especially in scenarios with high PPM order, small array pixel number and strong noise level. Besides, the nonlinear detection efficiency recovery model also reflects the best consistency with the experimental measurements of the SNSPD array in terms of response count rates near saturation.

Stratospheric aerosol lidar with a 300 µm diameter superconducting nanowire single-photon detector at 1064 nm.

Stratospheric aerosols play an important role in the atmospheric chemical and radiative balance. To detect the stratospheric aerosol layer, a 1064 nm lidar with high resolution and large dynamic range is developed using a superconducting nanowire single-photon detector (SNSPD). Measurements are typically performed at 1064 nm for its sensitivity to aerosol, whereas detectors are limited by low efficiency and high dark count rate (DCR). SNSPDs are characterized by high efficiency in the infrared wavelength domain, as well as low noise and dead time, which can significantly enhance the signal quality. However, it is still challenging to build an SNSPD with both large active area and high count rate. To improve the maximal count rate (MCR) so as to avoid saturation in the near range, a 16-pixel interleaved SNSPD array and a multichannel data acquisition system are developed. As a reference, a synchronous system working at 532 nm is applied. In a continuous comparison experiment, backscatter ratio profiles are retrieved with resolutions of 90 m/3 min, and the 1064 nm system shows better performance, which is sensitive to aerosols and immune to the contamination of the ozone absorption and density of molecule change in the lower stratosphere.

An On-Chip Photon-Counting Reconstructive Spectrometer with Tailored Cascaded Detector Array

The hybrid integration of superconducting nanowire single-photon detectors (SNSPDs) on various substrates and photonic structures has great potential on developing complicated photonic devices based on single-photon detections, such as photon-counting reconstructive spectrometers for the spectral sensing of single-photon level faint light. In this paper, we introduce the cascaded absorption effect of SNSPDs to develop a photon-counting reconstructive spectrometer. The device includes a Rowland grating as the spatial dispersion element and a tailored cascaded SNSPD array in the focusing region of the grating. The spectral responses of the SNSPDs could be flexibly modulated by their coiled patterns and the cascaded absorption in the array, which are used as the bases for spectral reconstruction. A prototype device was designed and fabricated to demonstrate the principle of the scheme. The experiment results showed the feasibility of the spectral response modulations by the coiled pattern design and the cascaded absorption effect of the SNSPD array. It supports the spectral measurement and reconstruction in the wavelength range of 1,495 to 1,515 nm, with a spectral resolution of 0.4 nm. The proposed scheme achieves the bases for spectral reconstruction only by the design of SNSPDs and without the spectral modulation effects of additional photonic structures. It provides an interesting and promising way to develop devices with high photon utilization.

Design and fabrication of single photon detector with ultra-large area superconducting nanowire array

<sec>Superconducting nanowire single-photon detector (SNSPD) is one of the most mainstream single-photon detectors at present, which possesses excellent comprehensive performance, including low time jitter, high efficiency, low dark count, and wide spectrum. However, the traditional single-pixel SNSPD suffers a lack of spatial resolution and a small photosensitive surface, which becomes a bottleneck associated with optical coupling efficiency. In addition, a single-pixel detector has no ability to resolving the photon number, whose working speed cannot be further improved due to the existence of dead time. While array devices can make up for the above deficiencies. Therefore, the development of a large-area SNSPD array is the key to free-space photon detection and other applications. In recent years, the relevant researches have been conducted and great progress has been achieved. However, the large-area SNSPD array is facing some intractable problems, including complex process, low yield, and difficult fabrication, owing to the photosensitive surface consisting of a large number of superconducting nanowires. Photons imaging is verified with this device. At present, in the existing studies mainly used is the three-dimensional technology with complicated process steps to fabricate large array SNSPDs. How to simplify the process has become a research focus.</sec><sec>In this work, we design an ultra-large area nanowire array structure and propose an innovative plane process. Taking advantage of the property that the electron beam resists HSQ (hydrogen silsesquioxane polymer) forming a silicon oxide electrical isolation layer after exposure, we fabricate a large array SNSPD with a simplified two-dimensional process and realize dimensionality reduction for the traditional three-dimensional process of a multilayer structure. By measurement in parallel, the devices enjoy high yield with no bad points found. In addition, a full-superconducting electrode is adopted in our design to reduce the thermal effect of resistors. We add series and parallel resistors in the pixels to divide the bias current evenly and expand the array scale optionally. At the same time, we also offer the design details of array SNSPDs, the related simulation of hot spots to verify the rationality of the design, the optimization of the preparation conditions of array devices, measurement scheme formulation, and other related work.</sec><sec>This work provides an idea for designing and fabricating ultra-large array SNSPD, which is expected to be applied to the fabrication of megapixel array SNSPDs. Combined with an efficient readout circuit, a focal plane photon detection and imaging system with both a large field of view and high sensitivity can be realized.</sec>

Design and fabrication of large-area superconducting nanowire single photon detector arrays

Superconducting nanowire single-photon detectors (SNSPDs) are currently one of the most mainstream single-photon detectors with excellent comprehensive performance, including low time jitter, high efficiency, low dark count and wide spectrum. However, the traditional single-pixel SNSPD suffers from a lack of spatial resolution and a small photosensitive surface, which creates bottlenecks associated with optical coupling efficiency. In addition, a single-pixel detector does not have the photon number resolution ability, whose working speed cannot be further improved due to the existence of dead time. In this case, array devices can make up for the above deficiencies. Therefore, the development of a large-area SNSPD array is the key to free-space photon detection and other applications. In recent years, experimental groups at home and abroad have started relevant research and have made great progress. However, the large-area SNSPD array faces intractable problems, including complex processes, low yield and difficult fabrication, owing to the photosensitive surface consisting of large amounts of superconducting nanowires. At present, the existing studies mainly use three-dimensional technology with complicated process steps to fabricate large array SNSPDs. How to simplify the process has become a research focus.<br>In this paper, we design a large-area nanowire array structure and propose an innovative plane process. Taking advantage of the property that the electron beam resists HSQ forming a silicon oxide electrical isolation layer after exposure, we fabricate a large array SNSPD with a simplified two-dimensional process and realize dimensionality reduction for the traditional three-dimensional process of a multilayer structure. By measurement in parallel, the devices enjoy high yield with no bad points found. In addition, a fully superconducting electrode is adopted in our design to reduce the thermal effect of resistors. We add series and parallel resistors in the pixels to divide the bias current evenly and expand the array scale optionally. At the same time, we also offer the design details of array SNSPDs, the related simulation of hot spots to verify the rationality of the design, the optimization of the preparation conditions of array devices, measurement scheme formulation and other related work.<br>This work provides an idea for the design and fabrication of ultra-large array SNSPDs, which is expected to be applied to the fabrication of megapixel array SNSPDs. Combined with an efficient readout circuit, a focal plane photon detection and imaging system with both a large field of view and high sensitivity will be realized.

Photon number resolvability of multi-pixel superconducting nanowire single photon detectors using a single flux quantum circuit

Superconducting nanowire single-photon detectors (SNSPDs) are typical switching devices capable of detecting single photons with almost 100% detection efficiency. However, they cannot determine the exact number of incident photons during a detection event. Multi-pixel SNSPDs employing multiple read-out channels can provide photon number resolvability (PNR), but they require increased cooling power and costly multi-channel electronic systems. In this work, a single-flux quantum (SFQ) circuit is employed, and PNR based on multi-pixel SNSPDs is successfully demonstrated. A multi-input magnetically coupled DC/SFQ converter (MMD2Q) circuit with a mutual inductance M is used to combine and record signals from a multi-pixel SNSPD device. The designed circuit is capable of discriminating the amplitude of the combined signals in accuracy of Φ 0/M with Φ 0 being a single magnetic flux quantum. By employing the MMD2Q circuit, the discrimination of up to 40 photons can be simulated. A 4-parallel-input MMD2Q circuit is fabricated, and a PNR of 3 is successfully demonstrated for an SNSPD array with one channel reserved for the functional verification. The results confirm that an MMD2Q circuit is an effective tool for implementing PNR with multi-pixel SNSPDs.

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.

Enhanced photon communication through Bayesian estimation with an SNSPD array

Laser communication using photons should consider not only the transmission environment’s effects, but also the performance of the single-photon detector used and the photon number distribution. Photon communication based on the superconducting nanowire single-photon detector (SNSPD) is a new technology that addresses the current sensitivity limitations at the level of single photons in deep space communication. The communication’s bit error rate (BER) is limited by dark noise in the space environment and the photon number distribution with a traditional single-pixel SNSPD, which is unable to resolve the photon number distribution. In this work, an enhanced photon communication method was proposed based on the photon number resolving function of four-pixel array SNSPDs. A simulated picture transmission was carried out, and the error rate in this counting mode can be reduced by 2 orders of magnitude when compared with classical optical communication. However, in the communication mode using photon-enhanced counting, the four-pixel response amplitude for counting was found to restrain the communication rate, and this counting mode is extremely dependent on the incident light intensity through experiments, which limits the sensitivity and speed of the SNSPD array’s performance advantage. Therefore, a BER theoretical calculation model for laser communication was presented using the Bayesian estimation algorithm in order to analyze the selection of counting methods for information acquisition under different light intensities and to make better use of the SNSPD array’s high sensitivity and speed and thus to obtain a lower BER. The counting method and theoretical model proposed in this work refer to array SNSPDs in the deep space field.

A Superconducting Binary Encoder with Multigate Nanowire Cryotrons.

Many classic and quantum devices need to operate at cryogenic temperatures, demanding advanced cryogenic digital electronics for processing the input and output signals on a chip to extend their scalability and performance. Here, we report a superconducting binary encoder with ultralow power dissipation and ultracompact size. We introduce a multigate superconducting nanowire cryotron (nTron) that functions as an 8-input OR gate within a footprint of approximately 0.5 μm2. Four cryotrons compose a 4-bit encoder that has a bias margin of 18.9%, an operation speed greater than 250 MHz, an average switching jitter of 75 ps, and a power dissipation of less than 1 μW. We apply this encoder to read out a superconducting-nanowire single-photon detector array whose pixel location is digitized into a 4-bit binary address. The small size of the nanowire combined with the low power dissipation makes nTrons promising for future monolithic integration.

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.

Receiver performance and detection statistics of single photon lidar

For a direct-detection lidar, the use of single photon detector (SPD) could greatly enhance the detection sensitivity of the lidar system. The insufficient SPD in dynamic range limits its application under strong background environment. The authors use the Neyman-Pearson detection theory to analyse the detection performance of single photon lidar under strong noise environment. As a result, the detection statistics can give a basic guide principle for lidar system design. The detection performance is related to input signal-to-noise ratio and the number of laser pulses that are integrated over. Furthermore, they analyse the influence of the dual detector AND gate, OR gate, and SUM module on detection performance. The SUM module, namely the SPD array, has the best detection performance. Based on this, a superconducting nanowire single-photon detector (SNSPD) array-based lidar is employed for field measurement. The array can make a pre-processing based on the array data in a single frame. It has a better detection performance for SNSPD array compared with that for single SNSPD.

Sixteen-Pixel NbN Nanowire Single Photon Detector Coupled With 300-μm Fiber

Niobium nitride (NbN) nanowires have a high repetition rate and efficiency, making them ideal for superconducting nanowire single photon detectors (SNSPDs). However, it is difficult to fabricate NbN arrays over large areas, which is critical for various applications. This paper describes a 4 × 4 NbN SNSPD array (16 pixels) and optical coupling with a 300-μm-diameter multimode fiber using beam compression technology. This is the first NbN SNSPD coupled with such large-diameter fibers. The designed pixels are positioned as closely as possible (pixel filling factor about 98.5%), almost without dead area between them. This results in a system efficiency of 46% and a quantum efficiency of 94.5% for photons (λ = 1064 nm) coupled from multimode fibers. An intrinsic time resolution of less than 69 ps can be obtained. The proposed high-performance single photon detector is suitable for satellite laser ranging. Furthermore, the proposed system is feasible for large SNSPD arrays with NbN, paving the way for the development of efficient photon cameras with NbN nanowires.

Suppressing Dark Counts of Multimode-Fiber-Coupled Superconducting Nanowire Single-Photon Detector

Large active-area superconducting nanowire single-photon detectors (SNSPDs) coupled with multimode fibers (MMFs) can provide high light-gathering capacity, which is essential for free-space detection applications in photon-starved regimes. However, MMF-coupled SNSPDs often suffer from large system dark count rates (DCRsys) over kHz due to blackbody radiation of the MMF at room temperature. Such large DCRsys would significantly degrade signal-to-noise ratio (SNR) of the receiving system. This paper reports an MMF-coupled large-active-area SNSPD system with low DCRsys by using a homemade cryogenic MMF filter bench. The filter bench, which consists of lenses and optical filters, can provide a high transmittance of about 80% at the central wavelength of the passband (1550±12.5 nm) and a wide blocking range from 500 nm to over 6000 nm at 40 K. With using the filter bench, the DCRsys of an MMF-coupled 9-pixel SNSPD array with an active area of 50 μm in diameter is greatly suppressed by 23 dB with 1 dB loss of system detection efficiency (SDE). The detector demonstrates an SDE of 51% at a DCRsys of 100 Hz for 1550 nm photons. Thus, SNR of the detector is enhanced by about 160 times and the noise equivalent power is improved to 3×10 $^{-19}$ W/Hz $^{1/2}$ .

A 16-Pixel Interleaved Superconducting Nanowire Single-Photon Detector Array With A Maximum Count Rate Exceeding 1.5 GHz

Increasing the count rate (CR) is one of the key issues in the development of superconducting nanowire single-photon detectors (SNSPDs) for different applications. This study aims to design, fabricate, and analyze a 16-pixel interleaved nanowire SNSPD array, which exhibits a system detection efficiency (SDE) of 72% and a dark CR of 100 Hz at a low-photon-flux limit and a wavelength of 1550 nm. By exploiting the increased pixel number and reduced dead time (<;5 ns) of each nanowire, the SNSPD array attained a maximum CR of 1.5 GHz with an SDE of ~12% at a photon flux of 1.26 × 10 10 photons/s. Moreover, the SNSPD achieved a photon number resolution of up to 16 photons.