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 (Tc), which characterizes the TESs: energy resolution scales with Tc1.5,, while signal fall time scales with Tc-3. Tc is optimized for specific applications. In quantum computing, fast detector response is critical, and moderate energy resolution is sufficient for distinguishing photon numbers. Therefore, Tc 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 Tc 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.
[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).
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