Single-photon detectors are common key technologies with significant application importance and research implications in frontier fields such as quantum information technology, single-photon laser communication/radar, and biological fluorescence spectral detection. In the past decade, with the rapid development of quantum information technology, superconducting nanowire single-photon detectors (SSPDs) have achieved nearly unity detection efficiency. Detection speed, and photon-number resolution are currently important technical bottlenecks and research directions in the field of optical quantum detection. Traditional single-photon detectors, such as avalanche photodiodes and photomultipliers, are difficult to meet current frontier application requirements. Although traditional single-unit SSPD has excellent performance, its detection rate (count rate) is usually only ~10 MHz due to limitations in readout circuit and detector recovery time. On the one hand, superconducting transition-edge sensors have good photon-number resolution and high detection efficiency, but have lower detection speed and larger time jitter, and require extremely low operating temperatures and complex readout cooling systems. Moreover, the photon-number resolution capability of single-unit SSPD is limited by the strong nonlinear transition from superconductivity to the normal state. It is difficult to distinguish photon numbers by using information such as response waveform. Multi-pixel SSPD is a novel device architecture that enables parallel operation of nanowires. Compared with single-pixel SSPD, multi-pixel devices have high efficiency and low dark count rates, and can enhance counting rate and photon-number resolution through parallel operation, providing an excellent solution for high-speed light detection and photon-number resolution.

Recently, the team of Hao Li and Lixing You from the Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences has developed a sandwich superconducting nanowire structure SSPD array, based on a 64-channel system compact GM cryocooler, achieving 90% system detection efficiency, 5.2 GHz maximum count and 61 photon number resolution in the 1550 nm communication band. Relevant research results were recently published in Photonics Research, Volume 12, Issue. 6, 2024. [Tianzhu Zhang, Jia Huang, Xingyu Zhang, Chaomeng Ding, Huiqin Yu, You Xiao, Chaolin Lv, Xiaoyu Liu, Zhen Wang, Lixing You, Xiaoming Xie, Hao Li, "Superconducting single-photon detector with a speed of 5 GHz and a photon number resolution of 61," Photonics Res. 12, 1328 (2024)]

Regarding the nanowire design, the NbN/SiO₂/NbN sandwich superconducting nanowire structure is utilized to decouple the optical absorption and photon response of the device. A dielectric mirror is used to achieve nearly ideal high detection efficiency. The device comprises 64 independently nanowires forming an SSPD array. Compared to a single SSPD structure with the same sensitive area, the dynamic inductance of the entire SSPD array is reduced to 1/64, effectively decreasing the recovery time of the device. Simultaneously, it exhibits photon number resolution due to the spatial distribution of the nanowires. With respect to the system integration, the project team has developed a 64-channel compact GM cryocooler operating at 2.3K. Additionally, they have constructed a multi-channel low-temperature readout test platform for real-time control, acquisition and processing of array SSPD devices.

Figure 1: Device structure (a), superconducting nanowire (b), device packaging (c), and refrigeration system (d).

Figure 2: Device detection efficiency (a) and the relationship between detection efficiency and count rate (b).

The measured yield of the superconducting nanowires is 61/64, and the detector demonstrates a system detection efficiency of 90% at 1550 nm, with a maximum detection speed of 5.2 GHz (Fig 2). Furthermore, the device achieves a photon-number resolution of 61 due to multi-wire parallel operation (Fig 3).

Figure 3: Graph showing the variation of photon number statistics with incident light intensity.

This work demonstrates a superconducting detector composed of 64 parallel nanowires with high speed, high efficiency and high photon number resolution. The detector performance can be further improved. For instance, replacing the AC coupling readout with a cryogenic DC coupling setup may lead to count rates exceeding 10 GHz. Furthermore, the photon number resolution fidelity may be refined by enhancing the SDE of nanowires or the number of nanowires. The detector system can be used in diverse applications, including deep-space laser communication, quantum information processing, quantum metrology, and other fundamental quantum optics experiments.