Takashi Noguchi

Origin of subgap current in an SIS junction

According to the BCS theory, the subgap current in an SIS junction is reduced exponentially as the temperature decreases. However, experimentally, the reduction is saturated below a certain temperature. This phenomenon was a bottleneck in the development of sensitive SIS photon detectors with NEP of 10−19u2009W/Hz at 650 GHz, because the NEP is limited by the shot noise in the subgap current. To investigate the origin of the residual subgap current under various temperatures we have measured I‐V curves of a Nb/Al‐AlN/Nb junction. Obtained I‐V curves have been reproduced by a model: single‐and two‐particle tunneling under reduced quasiparticle lifetime. Our result suggests that increasing the lifetime is essential to reduce the subgap current. One of possible methods to increase the lifetime is to form an epitaxial Nb film.

Superconducting THz Camera with GaAs‐JFET Cryogenic Readout Electronics

We describe the development of large format array of superconducting tunnel junction detectors that is readout by SONY GaAs‐JFET cryogenic integrated circuits. High quality SIS photon detectors have high dynamic impedance that can be readout by low gate leakage GaAs‐JFET circuits. Our imaging array design, with niobium SIS photon detectors and GaAs‐JFET cryogenics electronics, uses integrating amplifiers, multiplexers and shift‐registers to readout large number of pixels that is similar to CMOS digital cameras. We have designed and fabricated GaAs‐JFET cryogenic integrated circuits, such as AC‐coupled capacitive trans‐impedance amplifier, multiplexers with sample‐and‐holds and shift‐registers, for 32‐channel readout module. The Advanced Technology Center of National Astronomical Observatory of Japan have started extensive development program for large format array of SIS photon detectors.

Technical achievements of the ALMA future receiver development program at the National Astronomical Observatory of Japan

The ALMA telescope has been producing ground-breaking science since 2011, but it is mostly based on front-end and back-end technology from the 2000s. In order to keep ALMA competitive in the coming decade, timely updates are necessary in order to further improve the science output of the telescope. In NAOJ, we have been doing research leading to technological developments which aim to increase the field-of-view of the telescope, and the RF and instantaneous bandwidth for more efficient and accurate spectral surveys. In this contribution, we will describe the most important technical achievements by our group in recent years.

Development of a Superconducting Submillimeter-Wave Limb Emission Sounder (SMILES) on the Japanese Experiment Module (JEM) of the International Space Station (ISS)

NASDA and CRL are planning to develop a spaceborne SMILES, which is to be installed in the Exposed Facility (EF) on the JEM of the ISS. By observing gases such as ClO, HCl, NO, N2O, HO2 and BrO in the stratosphere, JEM/SMILES can trace the chemical reactions concerning the ozone depletion and climate change. Global distribution of those gases will be mapped with a height resolution of about 2 km. JEM/SMILES receives low-intensity signals from those gases with highly sensitive SIS (Superconductor-Insulator-Superconductor) mixers at 640 GHz, which are cooled to 4.2 K by a space-qualified mechanical cooler. The mission target is to demonstrate the effectiveness of the submillimeter-wave limb emission sounding and to establish space applicability of the low-noise SIS mixers and a mechanical 4-K cooler. JEM/SMILES is expected to be launched in 2003, and the experiments will last a year or more.