We designed, fabricated and studied the samples of the sub-terahertz oscillators based on shunted Josephson junction arrays. The junctions are embedded into the central electrode of the coplanar transmission line. Three series of samples with the tunneling current densities 5 kA/cm2, 13 kA/cm2 and 30 kA/cm2 were produced. The emission power at operating frequencies in range 400–700 GHz exceeds $0.1\mu \mathrm{W}$ for the junctions with AlOx tunnel barrier and $1\mu \mathrm{W}$ for AlN, which is already sufficient for the on-chip applications. The linewidth estimates yield the value less than 3 MHz in the best points, which enables the implementation of the phase-locking loop system.

Superconducting integrated circuits based on high-quality Nb–AlO x –Nb and Nb–AlN–NbN tunnel junctions, including a harmonic mixer with a high harmonic number, have been developed, optimized, and investigated. Details of the design, manufacturing methods, and specificities of operation of superconducting elements and circuits for the detection and study of terahertz radiation from cryogenic integrated oscillators are presented. The signal of an integral terahertz oscillator was detected, its power was estimated, and the radiation spectrum was measured with a resolution of about 1 Hz. The synchronization of a superconducting oscillator at any frequency in the range 250–750 GHz with a spectral quality higher than 50% has been realized, and the phase noise of the generator in the PLL mode has been measured.

The main parameters of the tunneling barrier of the Josephson junctions Nb/AlOx/Nb and Nb/AlN/Nb were estimated using the Simmons method in a wide range of current densities. The dependences of the height and width of the tunnel barrier on the resistivity for each type of junctions are experimentally determined. A decrease in the height of the AlN tunnel barrier by 0.3 eV, compared with the oxide one, makes it possible to obtain junctions with a current density above 15 kA/cm2 at a technologically achievable insulation layer of the order of 10 Angstroms, which makes it possible to realize the quality parameter Rj/Rn not lower than 25.

The main parameters of the tunnel barrier of Josephson Nb/AlOx/Nb and Nb/AlN/Nb junctions are estimated in a wide range of the current density using the Simmons method. The dependences of the tunnel barrier height and width of the resistivity are determined experimentally for each type of the junctions. A decrease in the tunnel barrier height of the junction with the AlN interlayer by 0.3 eV as compared to an oxide junction enables us to obtain the junctions with a current density higher than 15 kA/cm2 at an insulating layer thickness of 10 Å allowable technologically, which gives the possibility to obtain quality parameter Rj/Rn no lower than 25.

Coherent THz emission from stacks of intrinsic Josephson junctions (IJJs), created naturally in the Bi2Sr2CaCu2O8+x unit cell, was measured by a superconducting integrated receiver (SIR). The noise temperature of the SIR is as low as 120 K and its spectral resolution is better than 0.1 MHz, which exceeds the resolution of modern terahertz-range Fourier spectrometers by several orders of magnitude. In this report, the results of the spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks are summarized. The phase-locked SIR has been used also for the locking of the BSCCO oscillator under the test. About 10 % of the power emitted by the BSCCO oscillator operating at 563 GHz with free-running linewidth of 13.5 MHz has been phase locked. The possibility of mutual locking of two BSCCO oscillators fabricated on one substrate has been investigated by direct measurements of emitted radiation spectra by the SIR.

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency ${f}_{e}$ is continuously tunable via changing the bias current and the bath temperature, but at low bias ${f}_{e}$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.

A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature TC ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm (TC ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10–13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10–19 W/Hz1/2 at a temperature of 150 mK.

AbstractA high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T _ C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ( T _ C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10^–13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10^–19 W/Hz^1/2 at a temperature of 150 mK.

We present proof-of-operation for a new method of electron thermometry using microwave impedance of a hafnium micro-absorber. The new method leads to an ultimate THz-range detector suitable for microwave readout and frequency division multiplexing. The sensing part of the device is a hot-electron-gas absorber responding to the incident radiation by variation of its impedance measured at probing frequency about 1.5 GHz. The absorber is a microbridge made from hafnium (Tc = 375 mK, RN = 30 Ohm) sized 2.5 um by 2.5 um by 50 nm and integrated with a planar 600-700 GHz antenna placed near the open end of a quarter-wave CPW resonator (Q-factor about 10^4). All elements of the circuit, except the microbridge, are made from 100-nm thick Nb, including the resonator, which is weakly coupled to a throughput line. The device was tested at 50-350 mK smoothly responding with its transmission coefficient S21 to applied microwave power at the resonance frequency. We have found that the power absorbed by the bridge fits to the model of hot electron gas, P=k(Te^n-Tph^n) (n = 5...6). The idle NEP down to about 10^-18 W/Hz^(-1/2) and the corresponding cross-over temperature for photon background about 5 K are estimated from the measured data. The saturation power of about 1 pW and possibility of moderate gain are anticipated for a practicable device operating at temperature 200 mK. Since the optimum readout frequency is found exactly at the resonance, the detector is insensitive to most phase instabilities at the probing frequency.

We analyze the microwave-driven transition of a superconducting bridge into its normal state. The micro-bridge made from Hf is studied well below its critical temperature, Tc=375 mK, for a number of bath temperatures down to 30 mK. The bridge sized 2.5 {mu}m by 2.5 {mu}m by 50 nm was integrated near the open end of 1.5-GHz CPW quarter-wave resonator made from Nb yielding the Q-factor about $10^4$. The integrated circuit is designed for FDM readout of the bridge impedance and operates similar to MKID. We observe a smooth dependence of the Q-factor and S21, the transmission parameter of the chip, on applied microwave power. A novel method of steady state Q-factor is used for evaluating thermal conductance of the bridge. The microwave power absorbed by the bridge is found fitting the model of hot electron gas, $P=a({Te}^6-{Tph}^6)$, that allows to calculate thermal conductance of the bridge and evaluate NEP down to $10^{-18}$ W/Hz$^{1/2}$. This number estimated for the first experiment can be scaled down to and below $10^{-19}$ W/Hz$^{1/2}$ via reduction of both the bridge size and Tc of the hafnium film. Since it was found that the major part of the microwave impedance is active, the proposed detection technology is beneficial for reducing phase jitter in a high-Q resonator. According to our experimental data, a bridge made from Hf may operate as a THz signal sensor using readout frequencies above few GHz.

V.P. Koshelets, N.V. Kinev, A.B. Ermakov, F. Rudau, R. Wieland, D. Koelle, R. Kleiner, and H.B. Wang 1 Kotel’nikov Institute of Radio Engineering and Electronics RAS, Moscow 125009, Russia; valery@hitech.cplire.ru 2 Physikalisches Institut and Center for Collective Quantum Phenomena in LISA+, Universität Tübingen, D-72076 Tübingen, Germany; 3 Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, China.

We describe the first phase of experimental study of the superconducting bridge RFTES detector at temperatures 20-300 mK including the measurement of its thermal conductance, which fits the model of electron gas heating. We discuss the idea of the RFTES scheme, which is based on the probing of microwave loss near superconducting transition of the bridge. The heat applied to the bridge is generated by the probing signal at the frequency of the high-Q resonator. Since the real part is dominating in the nonlinear impedance of the bridge, the applied heat provides merely amplitude modulation of Q suggesting the suppression of phase jitter of the resonator. The bridge was made from a 50-nm-thick hafnium film (T-c approximate to 380 mK) sized to 2.5 mu m x 2.5 mu m. The resonator and the rest of the circuit were made from 200-nm-thick film of niobium (T-c approximate to 9 K) demonstrating the loaded Q-factor up to and above 10 000 at 1.5 GHz. A cryogenic semiconductor amplifier was used in the readout circuit. The thermal conductance was measured using the steady Q regime of the resonator and found to follow 76 down to and below G approximate to 1 x 10(-1)(3) W/K. The NEP below 10(-18) W/root Hz is estimated for the electron temperature of the bridge about 300 mK.

Our new detector and readout concept brings together TES (Transition Edge Sensor) and MKID (Microwave Kinetic Inductance Detector) technologies and exploits the idea of a microwave-induced superconducting transition in a small thin-film microbridge. The superconducting transition of the bridge manifests itself as variation in the Q -factor of niobium resonators at 5-8 GHz, somewhat similar to MKID operation. We present data showing the potential for developing this concept into multipixel detector arrays. Single-pixel sensitivity was measured at 4.5 K for an input band of 600-700 GHz using a prototype 10-nm-thick Nb bridge of size 1 μm × 500 nm. Radiation from human skin was detected with a resolution better than 1 K/rtHz, which is encouraging for terahertz imaging applications. To further improve device sensitivity, we are also developing Hf-based devices that operate near 0.35 K. Details about the physics and stability of these devices are discussed.

We describe the design, performance, and commissioning results for the new ALMA Band 5 receiver channel, 163–211 GHz, which is in the final stage of full deployment and expected to be available for observations in 2018. This manuscript provides the description of the new ALMA Band 5 receiver cartridge and serves as a reference for observers using the ALMA Band 5 receiver for observations. At the time of writing this paper, the ALMA Band 5 Production Consortium consisting of NOVA Instrumentation group, based in Groningen, NL, and GARD in Sweden have produced and delivered to ALMA Observatory over 60 receiver cartridges. All 60 cartridges fulfil the new more stringent specifications for Band 5 and demonstrate excellent noise temperatures, typically below 45 K single sideband (SSB) at 4 K detector physical temperature and below 35 K SSB at 3.5 K (typical for operation at the ALMA Frontend), providing the average sideband rejection better than 15 dB, and the integrated cross-polarization level better than –25 dB. The 70 warm cartridge assemblies, hosting Band 5 local oscillator and DC bias electronics, have been produced and delivered to ALMA by NRAO. The commissioning results confirm the excellent performance of the receivers.

A superconducting integrated receiver (SIR) (superheterodyne receiver) is developed for detection of radiation in a frequency range of 450–650 GHz. The SIR is used as a detector in the Terahertz Limb Sounder (TELIS) instrument that is mounted on a high-altitude balloon for the study of the atmospheric composition in the terahertz frequency range using limb sounding. The TELIS instrument and its detectors are described, and several results of the measurements in Polar Regions in 2009–2014 are presented

The Swedish-ESO PI receiver for APEX (SEPIA) was installed at the APEX telescope in 2015. This instrument currently contains ALMA Band 5 (157-212 GHz) and Band 9 (600-722 GHz) receivers. Commissioning and science verification for Band 5 have been successfully completed but are still ongoing for Band 9. The SEPIA instrument is briefly described and the commissioning of the Band 5 receiver and results from the first science observations are presented.

A superconducting integrated receiver (SIR) comprises all of the elements needed for heterodyne detection on a single chip. Light weight and low power consumption combined with nearly quantum-limited sensitivity and a wide tuning range of the superconducting local oscillator make the SIR a perfect candidate for many practical applications. For the first time, we demonstrated the capabilities of the SIR technology for remote operation under harsh environmental conditions and for heterodyne spectroscopy at atmospheric limb sounding on board a high-altitude balloon. Recently, the SIR was successfully implemented for the first spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks (BSCCO mesa) at frequencies up to 750 GHz; linewidth below 10 MHz has been recorded in the high bias regime. The phase-locked SIR has been used for the locking of the BSCCO oscillator under the test. To extend the operation range of the SIR well above 1 THz, a new technique for fabrication of high-quality SIS tunnel junctions with gap voltage Vg up to 5.3 mV has been developed. Integration of a superconducting high-harmonic phase detector with a cryogenic oscillator opens a possibility for efficient phase locking of the sources with free-running linewidth up to 30 MHz that is important both for BSCCO mesa and NbN/MgO/NbN oscillators.