With the advent of the carbon-neutral society, there is a growing need for underground exploration technology based on quantum sensing using high-temperature superconductors. In particular, deep and highly accurate electromagnetic surveys are required for exploration of hydrothermal reservoirs for geothermal power generation and monitoring technology for carbon dioxide capture and storage (CCS). This report describes the high-temperature superconducting SQUID technology and its role in the new society of green transformation (GX) and carbon neutrality.

Photon absorption in a superconducting strip with a thickness of only a few nanometers produces excited quasiparticles eventually produce a localized resistive region. A Superconducting Strip Photon Detector （SSPD） based on this phenomenon was proposed in 2001, and today, the detection efficiency is approaching 100 %, the dark count rate and timing jitter is overwhelmingly low as compared to other single photon detectors. SSPD has already positioned as an indispensable technology in various advanced technological fields such as quantum technology, live cell observation, LIDAR, and deep space optical communication technology. Recently, there have been several reports that break down stereotypes about what an SSPD should be. In this article, I focus on intriguing topics concerning the latest research and development trends in SSPD.

We developed the Low-Tc Superconducting Quantum Interference Device（LTS SQUID） magnetometer system to observe geomagnetic fields with high sensitivity. The system is comprised of a three-axis SQUID magnetometer housed in a glass fiber reinforced plastic（GFRP） cryostat, low-drift readout electronics with flux locked loop（FLL）, and a 27-bit data logger with a global positioning system（GPS） and telemetry function. All components operate with batteries. The SQUID magnetometers were electromagnetically shielded in liquid helium, using a cylindrical, copper shield. The system noise was approximately 25 fT/ √Hz in the 0.5 – 100 Hz frequency range, and the temperature drift of the system was ~1 pT/°C. The liquid helium boil-off rate of the system was 1.2 L/d, and the system could operate for 4 weeks. The system could detect weak geomagnetic signals, such as the Schumann resonance with eighth harmonics, and the Ionospheric Alfvén Resonance appearing at night. Installing two SQUID magnetometer systems at different places around 150 km apart, we performed comparative measurements of the geomagnetic fields successfully. We confirmed that the SQUID magnetometer system was useful for detecting the natural electromagnetic signals with high sensitivity in the extremely low frequency （ELF） range.

Superconducting Transition Edge Sensors （TESs） are promising optical photon detectors, which can count infrared photons with energy information. TES characteristics are explained in terms of a readout system with a negative feedback circuit. The current status of the optical TES detector development is introduced. We also introduce the development of our Ir-TES. Further optimization of the readout circuit is anticipated by using the recent fast cryo-electronics.

The cutting-edge superconducting detector, Microwave Kinetic Inductance Detector（MKID） has been popular in the field of astronomical radio observations such as Cosmic Microwave Background（CMB）, star-forming regions, and galaxy surveying. MKID is a resonator-based superconducting detector which has the scalability to exceed O（1,000） pixels in one readout line. The photon-noise-limited sensitivity can be achieved with the hybrid-type MKID which consists of two superconducting materials. The hybrid-type MKID arrays are utilized in “GroundBIRD” and “Tsukuba-KID camera” projects which are led by Japanese teams. Each project observed astronomical objects by the commissioning with developed MKID arrays and will start the science observations in the near future.