This paper describes the development of superconductor–insulator–superconductor (SIS) receivers in the 787–950 GHz radio frequency (RF) range, which covers the highest frequency band of the Atacama Large Millimeter/submillimeter Array (ALMA) and is recognized as the most difficult band in terms of superconducting technology, because the conventional superconducting material of Nb cannot be used for the circuitry in the mixer devices at the frequencies. The development began at the National Astronomical Observatory of Japan (NAOJ) in 2005, and the manufacturing and testing of all the receivers to be installed in the 66 Cassegrain reflector antennas that compose ALMA was completed in 2013. This enabled the terahertz frequency observations with the highest sensitivity from the ground. To meet the stringent ALMA requirements, terahertz SIS mixers with high‐quality superconducting NbTiN films were developed, which successfully demonstrated an unpreceded noise performance less than 230 K (5 times the quantum noise) for all the receivers. After the construction of ALMA, NAOJ began development studies for ALMA enhancement such as wideband and multibeam SIS receivers according to top‐level science requirements. To increase instantaneous bandwidth of the receivers, a submillimeter‐wave multiband receiver concept with a waveguide multiplexer, wideband intermediate frequency SIS‐mixer‐amplifier, and multifrequency local oscillator (LO) source is presented. The multibeam receiver employs a planar‐integrated SIS mixer circuit that includes all the RF components except the LO distribution network and an SIS‐mixer‐based amplifier of low power consumption, which is expected to enable the wide field‐of‐view observations in the future.

This paper describes the development of superconductor-insulator-superconductor (SIS) mixers for the Atacama Large Millimeter/submillimeter Array (ALMA) from the device point of view. During the construction phase of ALMA, the National Astronomical Observatory of Japan (NAOJ) successfully fabricated SIS mixers to meet the stringent ALMA noise temperature requirements of less than 230 K (5 times the quantum noise) for Band 10 (787-950 GHz) in collaboration with the National Institute of Information and Communications Technology. Band 10 covers the highest frequency band of ALMA and is recognized as the most difficult band in terms of superconducting technology. After the construction, the NAOJ began development studies for ALMA enhancement such as wideband and multibeam SIS mixers according to top-level science requirements, which are also presented.

We report on a 275–500 GHz heterodyne receiver system in combination with a wideband intermediate-frequency (IF) backend to realize 17 GHz instantaneous bandwidth. The receiver frontend implements a heterodyne mixer module that integrates a superconductor-insulator-superconductor (SIS) mixer chip and a cryogenic low-noise preamplifier. The SIS mixer is developed based on high-current-density junction technologies to achieve a wideband radio frequency (RF) and IF bandwidth. The IF backend comprises an IF chain divided into two channels for 4.0–11.5 GHz and 11.3–21.0 GHz and an analog-to-digital converter (ADC) module that is capable of high-speed sampling at 32 Giga samples per second with 12.5 GHz bandwidth per channel and an effective number of bits of 6.5. The IF backend allows us to simultaneously cover the full 4–21 GHz IF range of the receiver frontend. The measured noise temperature of the receiver frontend was below three times the quantum noise (hf/kB) over the entire RF band. A dual-polarization sideband-separating receiver based on this technique could provide up to 64 GHz of instantaneous bandwidth, which demonstrates the possibility of future wideband radio astronomical observations with advanced submillimeter-wave heterodyne receivers.

A compact 780-950 GHz sideband separating (2SB) superconductor-insulator-superconductor (SIS) mixer measuring 22 mm x 27 mm x 11 mm is designed in this study. In this mixer block, all components such as a radio frequency (RF) 90 degrees hybrid coupler, a local oscillator (LO) power splitter, two LO couplers, two identical SIS chips, and an intermediate frequency (IF) 90 degrees hybrid coupler are integrated. To minimize the waveguide length for the RF signal path, we separate the placement of the waveguide components into two layers in parallel. One layer contains the RF hybrid and LO couplers, and another layer contains the LO power splitter located above the RF hybrid coupler. They are connected by waveguides fabricated via wire electric discharge machining. We performed three-dimensional electromagnetic simulations and confirmed the results. Furthermore, a 4-12-GHz IF 90 degrees hybrid coupler to combine the IF signals from each SIS chip is designed with an alumina substrate having a relatively high dielectric constant to be integrated in the mixer block. The preliminary test result of single sideband noise temperatures of the fabricated 2SB SIS mixer partly complied with the current Atacama Large Millimeter/submillimeter Array (ALMA) specifications without any loss correction in front of the receiver. Because the RF and LO interfaces of the mixer block are the same as that of the current ALMA band 10 mixer block, band 10 cartridges are expected to be upgraded to 2SB configurations without significant changes in optics.

NAOJ have studied wideband receiver technologies at submillimeter wavelengths toward implementation as future upgrades into the Atacama Large Millimeter/submillimeter Array telescope. We have developed critical components and devices such as waveguide components and superconductor-insulator-superconductor (SIS) mixers targeting radio frequencies (RF) in the 275-500 GHz range and an intermediate frequency (IF) bandwidth of 3-22 GHz. Based on the developed components, quantum-limited low-noise performance has been demonstrated by using a double-sideband receiver frontend in combination with a high-speed digitizer. In addition, a preliminary demonstration of a wideband RF/IF sideband-separating SIS mixer was performed. This paper describes the status of our efforts to develop technology toward wideband receivers for ALMA.

Superconductor-insulator-superconductor (SIS) tunnel junctions based on Nb/Al-AlOx/Nb tri-layers (TL) [1] are the standard technology in superconducting electronics. This is owed to the development of a reliable fabrication process that produces junctions with current densities up to jc = 10 – 15 kA/cm2 and low sub-gap leakage. Nb/Al-AlNx/Nb TLs offer to make junctions with current densities as high as jc ~ 70 kA/cm2 while maintaining low sub-gap leakage [2] but its use has not yet become wide spread. The deposition process for both TLs follows the same approach, with one of the merits being that a high quality insulator can be formed on the thin Al film that covers the Nb base electrode. Junctions made with either barrier type exhibit qualitatively the same behavior: Sub-gap leakage increases with current density. This is commonly seen as a signature of non-uniformity as the barrier`s average thickness decreases. Since the invention of Nb SIS junction technology in the early 1980s, it has been known that TLs with a second thin Al film near the barrier, e.g. Nb/Al-AlOx/Al/Nb, thus making it a symmetric layer stack, produces junctions with higher resistance and lower subgap leakage [1]. Obtained with relatively low current density TLs, jc ~ 1 kA/cm2, the results are attributed to the protective function of the Al layer, preventing a chemical reaction between niobium counter electrode (CE) and barrier. This poses the general question whether barrier reduction due to interface chemistry and junction quality are correlated and more specifically whether the choice of a single Nb layer as the CE limits the possibility to realize high quality junctions with current densities jc > 10 kA/cm2. A systematic study to address this issue is ongoing and to that end, we have fabricated junctions based on SIS TLs with Nb/Al base electrodes, AlNx barriers formed by nitridation, and three different CEs: single layer Nb, bilayer Al/Nb, and single layer NbN [3]. Plasma conditions during the nitridation process are identical and time t is varied to realize a range of barrier transparencies. Preliminary results confirm observations made in [1]: A Nb CE appears to reduce the AlNx barrier resulting in substantially higher current densities than Al/Nb or NbN CEs, see Fig. 1. As for the subgap leakage levels, our results also confirm the general trend of increased sub-gap leakage towards higher current densities. However, a direct comparison between junctions with different CEs is meaningful only if their

The Advanced Technology Centre (ATC) of National Astronomical Observatory of Japan is developing microwave kinetic inductance detectors (MKIDs) for large-array pixel cameras for millimeter and sub-millimeter astronomy. We investigated single-crystal Nb thin layers to form superconducting microresonators. We compared the performances of MKIDs based on crystalline Nb structure and those based on polycrystalline Nb. We carried out the entire manufacture of the detectors in the ATC clean room. DC magnetron sputtering is used to grow single-crystal Nb films on r-plane sapphire substrates at an elevated temperature of 800 °C. The residual resistivity ratio (RRR) measured on these single-crystal Nb layers reached values ranging from 40 to 80. We made MKIDs with this crystalline Nb layer, and we measured internal quality factors of the detectors up to 106. The measurement of the noise power spectral density of these MKIDs gave a low value of − 95 dBc/Hz from 100 Hz to 100 kHz. The internal quality factor Qi and the fractional resonance frequency change δfr/fr of MKIDs with respect to the temperature variation are usually following the extended Mattis–Bardeen equations. However, we noticed a deviation from the theoretical prediction for temperature lower than 1 K (in our case). This deviation has already been observed on Al MKIDs and explained by a theory taking into account the Kondo effect and the kinetic inductance contribution. We demonstrated that our measurements on single-crystal Nb MKIDs are also in agreement with the same theory.

The increase of the instantaneous bandwidth of low-noise heterodyne receivers is a key aspect for creating new prospects in radio astronomy at millimeter and submillimeter wavelengths. For example, instruments with wider intermediate frequency (IF) bandwidth would offer better sensitivity and multiline spectral observations without changing the local oscillator (LO) frequency. Wideband technology with respect to radio frequency (RF) is also beneficial because the possibility to cover wide RF ranges with the same receiver offers new science cases, mostly related to accurate observations of multiple spectral lines with the same receiver calibrations. In addition, the wideband RF technology allows us to reduce the total number of receivers to cover a particular RF range, and thus, to simplify maintenance and operation of telescopes. So far, we have independently studied and developed wideband RF and IF technologies. The double sideband (DSB) receiver implementing wideband RF components showed about 2 to 3 times the quantum noise over the RF 275-500 GHz with the IF band of 4-8 GHz [1]; The SIS mixer-preamplifier module based on high current density SIS junctions demonstrates low-noise and flat gain over the 3–18-GHz IF range at local oscillator frequencies of 400-480 GHz [2]. Our aim in this study is to develop a wideband RF and IF receiver technology with the same SIS mixer. We have designed and fabricated a wideband RF and IF SIS mixer combining two technologies. As shown in Fig. 1, the preliminary result showed DSB receiver noise temperature below 3 times the quantum noise for the 3-22 GHz IF over the entire LO frequencies. Moreover, the noise temperature averaged over 3-22 GHz was almost comparable with the one for 4-8 GHz. In the symposium, we will present the current status and latest result of the wideband RF and IF SIS mixer.

We have prepared superconducting resonators using high-quality Nb thin films with RRR = 48 and studied their resonance characteristics in detail. It was observed that the measured internal quality factor Q i reached as high as 4×10 7 , which might be the highest value obtained among the Nb thin film superconducting resonators ever reported. It is demonstrated that the temperature dependence of the internal quality factors can be well explained by considering the temperature dependence of the residual resistance due to phonon scattering, electron-electron scattering, and the Kondo effect of the residual quasiparticle in addition to the extended Mattis-Bardeen theory. It is also found that the change of the resonance frequency of the Nb thin film resonator with respect to temperature can be well explained by considering the temperature dependence of both the Kondo effect and the kinetic inductance of the residual quasiparticles. These results strongly indicate that a finite number of quasiparticles are present in the superconducting gap of the high-quality Nb film even at a sufficiently low temperature of T/T c <; 0.15 and form a Fermi liquid state.

In this paper, we present an investigation of the contribution of quantum susceptance in capacitance measurements of superconductor-insulator-superconductor tunnel junctions. We directly measured the one-port S-parameter S-11 by utilizing a 4-K probe station in combination with a network analyzer. We observed a bias voltage dependence of the measured junction susceptance which demonstrates the contribution of quantum susceptance. Calculation results performed, based on a simple equivalent circuit model, were in excellent agreement with the measurements. Experiment and analysis methods suggested here enable the quantitative verification of the contribution of quantum susceptance and are an effective way of extracting the junction's geometric capacitance.

We propose the use of SIS junctions as a frequency up-converter based on quasiparticle mixing in frequency division multiplexing circuits for multi-pixel heterodyne receivers. Our theoretical calculation showed that SIS junctions have the potential to achieve positive gain and low-noise characteristics in the frequency up-conversion process at local oscillator (LO) frequencies larger than the voltage scale of the dc nonlinearity of the SIS junction. We experimentally observed up-conversion gain in a mixer with four-series Nb-based SIS junctions at the LO frequency of 105 GHz for the first time.

We have successfully demonstrated low-noise and wideband SIS mixers for the 275-500-GHz radio-frequency (RF) band, aiming to cover Atacama Large Millimeter/submillimeter Array band 7 (275-373 GHz) and band 8 (385-500 GHz) simultaneously. The waveguide SIS mixers integrate a multisection impedance transformer with a parallel-connected-twin-junction or single-junction circuits based on Nb/AlN/Nb trilayers with a critical current density of 31 kA/cm(2). In order to achieve the targeted broadband performance, we designed an asymmetric one-side waveguide probe placed across a reduced height waveguide with dimensions 100 mu m x 580 mu m. The shape of the waveguide probe was optimized to have nearly constant impedance at around 35 Omega in the desired RF range. For evaluation of the receiver performance, we have established a dual-band noise temperature measurement system. The system implements a broadband corrugated horn antenna and a 15-dB RF/local oscillator (LO) waveguide coupler based on WR-2.3 (290 mu m x 580 mu m) waveguide size. The target bandwidth is too wide for typical LO sources at these high frequencies, and, therefore, we applied a dual-band LO injection system utilizing two different LO sources and an LO diplexer. The IF chain uses a cryogenic isolator and a low-noise amplifie with a typical noise temperature of 2.1 K over the 4-8-GHz range. The measured double sideband receiver noise temperature is two to three times the quantum noise at all LO frequencies for both the twin-junction and the single-junction circuits.

In this paper, we report on the design and performance of a prototype SIS mixer covering ALMA band-7 (275-373 GHz) and band-8 (385-500 GHz). The waveguide SIS mixer is based on a parallel connected twin-junction circuit with a current density of about 25 kA/cm2. For the DSB mixer evaluation system, a wideband corrugated horn antenna and a 15-dB RF/LO coupler have been designed and fabricated. LO sources for such a broad frequency coverage are currently not available, therefore, a dual band LO injection system with two LO sources and an LO diplexer have been used. The measured DSB receiver noise temperature ranges from 45 to 90 K and is compliant with existing ALMA band 7 and 8 receiver noise temperature specifications.

This paper reports the receiver performance based on a low-noise heterodyne module with very wide intermediate frequency (IF) bandwidth in the radio frequency range 385–500 GHz. The module integrates a superconductor-insulatorsuperconductor (SIS) mixer with a 3–21 GHz low-noise preamplifier. We utilize high current density junctions for the SIS mixer to achieve good matching conditions between the SIS junction and the amplifier, and to maintain the IF performance over the designed local oscillator (LO) frequencies. The measurement results of the receiver using the heterodyne module demonstrate a typical noise temperature of 70-80 K over 3–18 GHz, at LO frequencies of 400–480 GHz.

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.

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.