Takafumi Kojima

Three Quanta Sensitivity Superconductor-Insulator-Superconductor Mixer for the 0.78-0.95 THz Band

We have successfully demonstrated a low-noise 0.78-0.95-THz waveguide mixer based on Nb/AlO(x)/Nb superconductor-insulator-superconductor (SIS) junctions integrated with an NbTiN/SiO(2)/Al superconducting inverted microstrip. The mixer employs a twin-junction tuning circuit and an impedance transformer which are designed to achieve both low noise and wideband operation. Corrected for optical input losses in front of the mixer, the double-sideband (DSB) noise temperature is below 120 K (< 3 hf/k(B)) around the local oscillator (LO) frequency of 0.88 THz and at an operating temperature of 2 K. The results represent state-of-the-art performance for mixers in the terahertz frequency range

A sensitive ALMA Band 10 SIS receiver engineering model

We have developed a single polarization engineering model of the Atacama Large Millimeter/Submillimeter Array (ALMA) Band 10 (0.78-0.95 THz) receivers. The front-end optics comprises a pair of ellipsoidal mirrors and a corrugated feed horn. A waveguide mixer block is attached to the feed horn in which an NbTiN-based superconductor-insulator-superconductor (SIS) mixer chip, which uses a quartz substrate, is mounted onto a WR-1.2 full-height waveguide. The SIS mixer employs two Nb/AlO(x)/Nb junctions and an NbTiN/SiO(2)/Al microstrip tuning circuit. A very wide intermediate frequency (IF) system with a bandwidth of 4-12 GHz is employed. The receiver demonstrated double-sideband (DSB) noise temperatures of below 240 K at local oscillator (LO) frequencies ranging from 792 to 945 GHz, without any correction for loss in front of the receiver. The lowest DSB receiver noise temperature, 179 K, was obtained at the center frequency of the designed band, which corresponds to about 4 quanta. These results represent state-of-the-art sensitivity for a receiver at an operating physical temperature of 4 K.

Performance of Terahertz Waveguide SIS Mixers Employing Epitaxial NbN Films and Nb Junctions

We have designed, fabricated, and tested terahertz waveguide superconductor-insulator-superconductor (SIS) mixers with Nb/AlOx/Nb junctions and NbN/SiO2/Al microstriplines. The NbN ground plane used was an epitaxial film grown on a single-crystal MgO substrate. A two-junction tuning circuit, which was directly placed at the feed point of a bow-tie waveguide probe without impedance transformers, was used. A full-height waveguide, hammer-type choke filter, and zero-depth backshort were adopted for the design of the microstrip-waveguide transition structure. The Nb junctions in which the current density was 6.5 kA/cm2 showed good I-V characteristics, yielding a subgap-to-normal-state resistance ratio greater than 20. The results obtained for the mixer performance showed a receiver noise temperature of 440 K (DSB) at 1 THz; the results were corrected for losses in the beam splitter and the vacuum window. A detailed analysis of the mixers suggested that the waveguide mixers composed of epitaxial NbN films and Nb junctions were effective in the terahertz band.

Low-noise waveguide SIS mixer with NbN/AlN/NbN tunnel junctions tuned by an NbN/MgO/NbTiN microstrip circuit

The low-noise performance of a waveguide SIS mixer with NbN/AlN/NbN tunnel junctions and an NbN/MgO/NbTiN tuning circuit on an MgO substrate was demonstrated at frequencies above the gap frequency of Nb. To design the mixer, the complex refractive index of the MgO substrate was measured by using a THz time-domain spectrometer. The superconductive properties of NbTiN films on MgO were examined in detail. An uncorrected receiver noise temperature of 262 K (DSB) at 820 GHz was achieved in an IF bandwidth of 4-12 GHz at a bath temperature of 4.2 K. By comparing the measured and calculated conversion gains, the loss in the NbN/MgO/NbTiN tuning circuit was estimated to be 0.98 dB at 810 GHz.

Overview of the East Asia ALMA development program

ALMA has already produced many impressive and scientifically compelling results. However, continuous technical upgrades and development are key for ALMA to continue to lead astronomical research through the 2020-2030 decade and beyond. The East Asia ALMA development program consists of the execution of short term projects, and the planning and initial studies for longer term developments that are essential for future upgrades. We present an overview of all these ongoing East Asia ALMA development projects and upgrade studies, which aim to maintain and even increase the outstanding scientific impact of ALMA in the near future and over the coming decades.

How do we design the interferometric system focused on the analog and digital backend and the correlator for scientifically valuable ALMA developments

ALMA has been demonstrating its exceptional capabilities with unprecedented scientific results achieved over the past six years of operation. To keep ALMA as a leading-edge telescope, it is essential to continue technical upgrades and development of new potential. While our future development programs have already achieved remarkable technological breakthroughs at the level of front-end receivers, we are discussing the upgrades of the analog and digital backend and the correlator. We report the required concept design of the interferometric system focused on these sub-systems to realize new science use cases.

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.