Abstract Body

Single-photon detection technology has overcome the limitations of traditional photodetectors, achieving detection sensitivity close to the quantum limit. Single-photon detector serves as a key device for quantum information technology and promotes the development of many applications that depend on light or photon, such as quantum communication, quantum computing, laser communication, LIDAR, and imaging. In recent years, research on superconducting nanowire single-photon detectors (SNSPDs) has evolved from initially pursuing improvements in basic performance such as single-photon detection efficiency to developing large-scale imaging arrays. Some performance bottlenecks of the detectors have also emerged. This report will introduce the recent progresses made by Nanjing University in the development of superconducting single-photon detectors and imagers, as well as related implementations of these devices in single-photon communication, spectral detection, active/passive imaging, and integrated optical quantum chips.

Abstract Body

Transition-edge sensors (TESs) are superconducting single-photon detectors capable of resolving the energy of a single photon by detecting the slight temperature change caused by photon absorption. These detectors are designed and optimized to detect photons with specific energies, ranging from gamma-rays, X-rays, ultraviolet, visible, to mid/near-infrared wavelengths. Notably, Optical TESs have demonstrated energy resolution capable of distinguishing the number of incident photons at telecommunication wavelengths. They can also accurately measure the energy of a single photon. These capabilities make TESs suitable for applications such as quantum computing, biological imaging, and light dark matter search. In this talk, we will present recent progress in our development of optical TESs for these applications.

In theory, TESs involve a trade-off between energy resolution and detector response speed. TESs utilize the sharp resistance transition at the critical temperature (T_c), which characterizes the TESs: energy resolution scales with T_c^1.5,, while signal fall time scales with T_c^-3. T_c is optimized for specific applications. In quantum computing, fast detector response is critical, and moderate energy resolution is sufficient for distinguishing photon numbers. Therefore, T_c is typically set around 300 mK, resulting in an energy resolution of 100 to 200 meV and a fall time in the sub-ms range.

In biological imaging and light dark matter searches, high energy resolution is crucial, while the requirement for detector speed is more moderate. To explore the upper limits of TES energy resolution, we conducted tests on an Au/Ti (10 nm/20 nm) bilayer TES. By lowering the critical temperature Tc to 115 mK, we achieved a remarkable energy resolution of 67 meV full width at half maximum (FWHM) at 0.8 eV (1550 nm) [4]. The theoretical resolution, considering the typical energy resolution of optical TESs (150 meV) at Tc of 300 mK, would scale up to 30 meV FWHM when T_c is reduced to 115 mK. To investigate the discrepancy between the theoretical expectation and the measured value, we conducted measurements of the complex impedance and identified the thermal model for the TES as a two-block model. Parameters necessary for calculating the current noise were extracted from the measured complex impedance, and a comparison was made between the calculated and measured current noise of the TES.

To make TESs appealing for various applications, they should be arranged in arrays, multiplexed, and read out using a single cable. One challenge in reading out optical TESs is their fast detector response. Signals with sub-ms fall times require multiplexing at much higher carrier frequencies than the TES signal bandwidth, making microwave readout suitable. A microwave SQUID multiplexer meets this requirement. We designed and fabricated a 40-channel microwave SQUID multiplexer that operates with 5 MHz flux-ramp modulation, corresponding to the signal sampling speed [5]. Using this new multiplexer, we successfully measured signals from five pixels simultaneously and resolved photon-number peaks for three of the pixels. The energy resolution achieved ranged from 0.6 to 0.9 eV at 0.8 eV (1,550 nm) photons.

References

[1] B. Cabrera et al., Opt. Express 31, 12865-12879 (2023).
[2] L. S. Madsen et al., Nature, 606, 75–81 (2022).
[3] K. Niwa, K. Hattori, and D. Fukuda, Front. Bioeng. Biotechnol., 9, 789709 (2021).
[4] K. Hattori et al., Supercond. Sci. Technol. 35, 095002 (2022).
[5] R. Hayakawa et al., J. Low Temp. Phys. 215, 170 (2024).

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP20K04610, JST CREST Grant Numbers JPMJCR2004, JST-FOREST Program Grant No. JPMJFR2236 and World Premier International Research Center Initiative (WPI), MEXT, Japan. Transition-edge sensors and microwave SQUID multiplexers were fabricated at Superconducting Quantum Circuit Fabrication Facility (Qufab) the National Institute of Advanced Industrial Science and Technology (AIST). A part of this work was conducted at the AIST Nano-Processing Facility.

Keywords: Transition-edge sensor, single photon detector, microwave SQUIDs

Abstract Body

Magnetic microcalorimeters (MMCs) have become one of the superconducting detector technologies that demonstrate extreme energy sensitivity. MMCs utilize noble properties and advancements in superconducting sensor and electronics technology. In the presentation, the working principle of MMCs is covered in terms of the precise measurement of magnetic signals resulting from energy absorption in a detector using a superconducting circuit and a current-sensing SQUID. This presentation also highlights their best performance in the keV and MeV energy regions. Additionally, we will discuss various applications that utilize MMC as the primary detector technology.

Keywords: Magnetic Microcalorimeters, Superconducting detectors, Dark matter, Neutrino,

Abstract Body

Temporal and spatial detection of single photons has driven advances in single-photon imaging, fluorescence lifetime analysis, remote optical communication, and quantum information processing. Superconducting nanowire single-photon detectors (SNSPDs) have shown outstanding advantages in terms of efficiency, jitter, and speed. However, limited to the low-temperature environment, it is difficult to read out large-scale SNSPD arrays. Existing readout structures typically separately use dimensionality reduction (row-column architecture), encoding, and feature multiplexing (time or frequency) methods, which are challenging to further reduce readout complexity and increase pixel number. Combined with the strategies of dimensionality reduction and feature multiplexing, we designed an imaging sensor based on time-amplitude two-dimensional joint multiplexing, which can read out 1024 spatial positions with only two readout ports. Furthermore, by exploring the delay-time logic, the resolution of two-photon positions in 16 pixels is successfully realized. The imaging device has the characteristics of strong scalability and high pixel readout fidelity.

References

[1] Kong, L.-D. et al. Readout-efficient superconducting nanowire single-photon imager with orthogonal time–amplitude multiplexing by hotspot quantization. Nature Photonics 17, 65-72 (2023).

Figure 1. Superconducting nanowire two-photon coincidence counter (SNTPC). a, Schematic illustration of the device design and operation principle. b,c,d, A scanning electron micrograph of the 16 detector pixels and partial delay lines.

Keywords: Superconducting nanowire, single-photon imaging, two-photon detection

Abstract Body

We report on thermal effects induced superconducting nanowire arrays fabrication based on nano laser direct writing (NLDW). The superconducting nanowire arrays are fabricated by growing of 20 nm-thick Nb films via sputtering deposition on high-resistivity silicon substrates. Nano patterning is performed by optical lithography and followed by nano laser direct writing. By harnessing the advantages such as cost-effectiveness, efficiency, and design flexibility of NLDW, large-scale superconducting nanowire arrays up to 190 μm × 190 μm fabrication process was demonstrated. Furthermore, the thermal effect induced the degradation of Nb films was analyzed with simulated temperature distribution, electron probe x-ray microanalysis, and electrical transport measurements respectively by shrinking the spacing of nanowires step by step. Compared with the traditional micro-nano fabrication techniques such as electron beam and ion beam processes, the NLDW technique could provide a reliable, cost-effective, and reproducible pathway for scaling up superconducting circuits as well as an avenue to conduct nanoscale thermal engineering on superconducting materials studies for basic science.

References

[1] Natarajan CM, Tanner MG, Hadfield RH. Superconducting nanowire single-photon detectors: physics and applications. Supercond Sci Technol, 2012, 25: 063001.

[2] Holzman I, Ivry Y. Superconducting Nanowires for Single-Photon Detection: Progress, Challenges, and Opportunities. Adv Quantum Technol, 2019, 2: 1800058.

[3] Orús P, Sigloch F, Sangiao S, et al. Superconducting Materials and Devices Grown by Focused Ion and Electron Beam Induced Deposition. Nanomaterials, 2022, 12: 1367.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant No. 12104112), and the Natural Science Foundation of Shandong Province (Grant No. ZR2021QA036).

Figure 1. Schematic of NLDW system components. (a) Schematic of the NLDW system: The red laser is used for focusing, while the blue laser (405 nm) is used for writing. (b) Laser etching of nanowire arrays with different spacing on Nb film.

Figure 2. Heat transfer simulation of laser acting on Nb film. (a) Schematic of NLDW on Nb film to fabricate a nanowire array. (b) YZ and XZ sections indicate laser path. (c) Temperature distribution at different times in the YZ-plane. (d) Variation of XZ-plane temperature distribution with spacing between nanowires. (e) Temperature distribution versus time in the YZ-plane. (f) Temperature distribution of nanowires with a spacing of 600 nm versus time in the XZ-plane.

Figure 3. Optical microscope and SEM images of nanowire arrays. (a) 190 μm × 190 μm nanowire array fabricated by NLDW system. (b-c) The SEM images of nanowire arrays.

Figure 4. Characterization of electrical transport properties of Nb nanowire arrays. (a) I-V characteristics of the microbridge without NLDW at different temperatures (from 4.0 K to 8.0 K). (b) Normalized resistance versus temperature for different spacing of nanowires. (c) I-V characteristics of nanowire arrays with different spacing. (d) The relationship between Jc and the spacing of nanowire arrays.

Keywords: Superconducting nanowire, laser direct writing, Thermal effect