Primordial gravitational waves (PGWs) can only be detected by measuring the B-mode polarization of the cosmic microwave background (CMB) and large-scale arrays based on superconducting transition-edge sensors (TES) are the detectors of the choice thanks to the background-limited sensitivity. Different multiplexing readout techniques have been developed to combine the signals from multiple detectors into a reduced number of readout channels. Microwave SQUID multiplexing (μMUX) has the highest multiplexing factor, suitable for large-scale TES array readout. We are developing μMUX chips to read out TES array for the detection of PGWs. We have designed and fabricated a prototype of a 5-channel μMUX chip consisting of distributed quarter-wavelength coplanar waveguide (CPW) resonators and radio-frequency superconducting quantum interference devices (RF SQUIDs). The measured quality factor of the CPW resonators working at 4.7-4.9 GHz is higher than 104. The maximum frequency shift is about 290 kHz on average, compatible with the resonator bandwidth (~380 kHz). The mean value of magnetic flux noise is 1.1 μ$\Phi_0 / \sqrt{\mathrm{Hz}} @ 5$ kHz, corresponding to a current noise of 40 $\mathrm{pA} / \sqrt{\mathrm{Hz}}$.

Superconducting transition-edge sensors (TESs) are highly sensitive detectors and can detect electromagnetic wave radiations from millimeter/submillimeter, optical to 𝑥/γrays, suitable for cosmology, astrophysics, quantum information, and biosensing. In principle, thousands of TESs even more are required to enhance the detection efficiency for large-scale survey. Among other multiplexing schemes, microwave SQUID multiplexer (μMUX), consisting of resonators and RF SQUIDs, has a bandwidth of several GHz, thus multiplexing factor on the order of thousands, more suitable for readout of large TES arrays. We designed and fabricated superconducting coplanar waveguide (CPW) resonators with a high qualityfactor and second-order gradient RF SQUID with two inductive coupling structures respectively. Then, we optimized the critical current density of the Josephson junction and measured the mutual inductance parameters of the second-order gradient structure SQUID, which are consistent with the simulation results. Finally, we fabricated a cryogenic μMUX chip based on RF SQUID and resonator. We discussed the results of the development of μMUX in more detail.

The study of the origin and transport of water in the universe is an important part of the scientific program of the Millimetron space observatory. This will be made possible by observations conducted in single-dish mode using an onboard instrument-the high-resolution spectrometer (HRS). This instrument incorporates heterodyne array receivers operating within the range 0.5 -2.7 THz, comprising 3-pixel arrays of superconductor-insulator-superconductor mixers operating at frequencies below 1.3 THz and 7-pixel matrix receivers based on NbN HEB mixers observing above 1.3 THz. This article presents the current status of development for a mixers planned for use in the HRS instrument of the Millimetron space observatory.

Over the last two decades, the superconducting qubits have become a promising and rapidly growing area of research. However, there are specific challenges when working with such a delicate system. One of them is the qubit state readout. Since a very small signal (of the order of 1 photon) is used for the readout, its detection is challenging and it requires low-noise cryogenic amplifiers working in the 3–12 GHz frequency range. Josephson parametric amplifiers (JPAs) are one of the most studied and developed candidates. Currently, several JPA designs offer low noise performance and instantaneous bandwidth from tens of MHz to several GHz; they are also becoming an integral part of circuit quantum electrodynamics (cQED) setups. In the current work, results are presented of the measurements of a JPA made of a coplanar waveguide with an array of three SQUIDs. Nondegenerate amplification and an amplifier added noise being close to the quantum limit are demonstrated.

The Millimetron Space Observatory will be equipped with the cryogenically cooled instrument for a Space-Earth Very Large Baseline Interferometry (S-E VLBI). It will be a multi-channel heterodyne receiver with 7mm, 3mm, 1.3mm and 0.8mm channels. The VLBI instrument will have a multi-frequency capability, provided by signal splitting in the input optics and by back-end functionalities. The optical design is shown in this report.

In this work we have measured the frequency response of a heterodyne receiver based on a mixer at the Superconductor-Insulator-Superconductor (SIS) tunnel junction for the frequency range 211–275 GHz. The measurement was done with a Fourier spectrometer using two methods of reading the detector signal: by direct current and by the power of the intermediate frequency output signal. The reason for studying these methods is that intermediate frequency (IF) measurements give a better signal-to-noise ratio than direct current (DC) measurements. It is found that the spectrograms obtained by these methods are different up to the degree of squaring, also for IF measurements the appearance of artefacts in the microwave range, which coincide with the bandwidth of the IF system, is found. Modelling of the spectrometer operation for both methods is carried out, explaining the reason of 'artefacts' occurrence at measurements in the intermediate frequency reading mode and describing the nature and size of the convolution of the receiver spectrum in comparison with direct current measurements.

In this paper, we developed and fabricated a microcircuit based on “superconductor-insulator-superconductor” (SIS) junction as the detector of terahertz (THz) emission. Using this circuit, a signal in a range of 1-1.1 THz from backward wave oscillator (BWO) was detected. Pumping of the SIS detector by a signal with frequency higher than 1 THz lead to significant suppression of the gap voltage $V_{g}$ of the junction due to heating, which is not observed for frequencies lower than ~750 GHz. We proposed a model of the photon-assisted heating which could be useful for numerical calculations of parameters of the SIS junction affected by external emission. According to this model, a significant impact on the electron distribution function arises from a term responsible for Cooper pair breaking in the junction electrodes, which becomes relevant only at frequencies higher than twice gap frequency of the electrode material.

Suitably patterned single crystals made of the cuprate superconductor Bi2Sr2CaCu2O8+x (BSCCO), intrinsically forming a stack of Josephson junctions, can generate electromagnetic radiation in the lower terahertz regime. Because of Joule heating, the emission power of single stacks seems to be limited to values below 100 mu W. To increase the radiation power, mutually synchronized arrays situated on the same BSCCO base crystal have been studied. A maximum power of almost 1 mW has been achieved by synchronization of three stacks. Mutual electromagnetic interactions via a connecting BSCCO base crystal have been considered essential for synchronization, but the approach still suffers from Joule heating, preventing the synchronization of more than three stacks. In the present paper we show, on the basis of two emitting stacks, that mutual synchronization can also be achieved by standalone stacks contacted by gold layers and sharing only a common gold layer. Compared with BSCCO base crystals, the gold layers have a much higher thermal conductivity and their patterning is not very problematic. We analyze our results in detail, showing that the two oscillators exhibit phase correlations over a range of +/- 0.4 GHz relative to their center frequencies, which we studied mainly between 745 and 765 GHz. However, we also find that strong phase gradients in the beams radiated from both the mutually locked stacks and the unlocked stacks play an important role and, presumably, diminish the detected emission power due to destructive interference. We speculate that the effect arises from higher-order cavity modes that are excited in the individual stacks. Our main message is that the mutual interaction provided by a common gold layer may open new possibilities for relaxing the Joule-heating problem, allowing the synchronization of a higher number of stacks. The approach may also allow one to synchronize several stacks that are comparatively small in size and less prone to the strong phase gradients we observed. Our findings may boost attempts to substantially increase the output power levels of BSCCO terahertz oscillators.

In this article, we present the results of the research aimed at improving the fabrication process of a SIS-mixer for operation at frequencies close to 1 THz. We study the impact of buffer aluminum oxide layer and anodization effects on the properties of superconducting NbTiN films which form the electrodes of the transmission lines in THz-range devices. These layers are traditionally used in technological processes. The measurements of THz response are performed using terahertz time-domain spectrometer at frequencies from 0.3 to 2.0 THz. It was found that the critical temperature, normal-state conductivity just above the transition temperature, superconducting gap value and London penetration depth of the NbTiN film sputtered on aluminum oxide buffer layer are almost the same as of the film sputtered directly onto the substrate. The difference between the parameters is comparable to the measurement uncertainty for NbTiN films with and without additional surface layers of aluminum and anodization.

We performed the spectral measurements of a niobium based Josephson junction array in order to estimate the linewidth of Josephson emission. The array is formed by 9996 serially connected Nb/NbSi/Nb Josephson junctions occupying the area of 5 × 7 mm 2 on a silicon substrate and divided into 7 distinct subarrays. The array was immersed in a liquid helium dewar from where the emission was brought to open space via an oversize waveguide. Firstly, the measurements on a Fourier-transform spectrometer were carried out in a wide frequency range 139–343 GHz at almost all self-induced steps of current-voltage curve. As in our previous work, the observed linewidth of Josephson emission corresponded to the resolution of the spectrometer. Then, we carried out more precise and sensitive measurements using the 211–275 GHz heterodyne receiver based on a Nb/AlO x /Nb mixer with a spectral resolution better than 0.1 MHz. Dependencies of the linewidth on the step number and on the number of connected subarrays are studied. The peaks corresponding to the 2nd harmonic of the Josephson generation are also observed in the spectra. The linewidth of the main harmonic down to 1.5 MHz was observed in these measurements.

The terahertz band, a unique segment of the electromagnetic spectrum, is crucial for observing the cold, dark universe and plays a pivotal role in cutting-edge scientific research, including the study of cosmic environments that support life and imaging black holes. High-sensitivity superconductor–insulator–superconductor (SIS) mixers are essential detectors for terahertz astronomical telescopes and interferometric arrays. Compared to the commonly used classical Nb/AlO x /Nb superconducting tunnel junction, the Nb/AlN/NbN hybrid superconducting tunnel junction has a higher energy gap voltage and can achieve a higher critical current density. This makes it particularly promising for the development of ultra-wideband, high-sensitivity coherent detectors or mixers in various scientific research fields. In this paper, we present a superconducting SIS mixer based on Nb/AlN/NbN parallel-connected twin junctions (PCTJ), which has a bandwidth extending up to 490 GHz–720 GHz. The best achieved double-sideband (DSB) noise temperature (sensitivity) is below three times the quantum noise level.

At present, microwave transmission lines are characterized by a strong frequency dependence of loss in the subterahertz range. This work is aimed at development, research, and optimization of superconducting integrated circuits designed for matching the impedances of a long Josephson junction oscillator (a so-called “flux-flow oscillator”) and a superconductor–insulator–superconductor (SIS) detector in the subterahertz frequency range. The goal of this study is to improve and approve the numerical calculation methods, which make it possible to describe correctly experimental superconducting structures in a wide frequency range. Numerical calculations of integrated circuits have been performed in order to optimize the topology and parameters of transmission lines. The main parameters of the transmission lines and their influence on the signal propagation are determined. The results of optimization of integrated matching circuits in the range of 450–700 GHz have been experimentally confirmed. Optimization and improvement of transmission lines allow one to design new-generation integrated superconducting detectors and investigate tunnel SIS junctions more thoroughly (including shunted ones) and the properties of heterodyne oscillators based on long Josephson junctions.

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 structures are simulated in a frequency range of 300–750 GHz using two methods: (i) ABCD matrices related to each element of the circuit and (ii) Ansys HFSS software. The surface impedance of superconducting films is numerically calculated using expressions from the Mattis–Bardeen theory. For samples with microstrip line widths of less than one quarter of the wavelength, both models are in qualitative agreement with each other and with experimental data. It is shown that an increase in the width of the lines and the geometric dimensions of other circuit elements leads to generation of transverse modes and non-plane wave front of waves propagating along the lines, which causes discrepancy between the semi-analytical and numerical calculations, while the latter are in agreement with the experiment for all samples.

We present the results of studying the reflectivity of thin Nb and NbTiN films deposited on silicon substrates in the frequency range 205–255 GHz at temperatures from 5 K to room temperature. The experiment was performed using a resonator spectrometer, in which the studied specimen is a mirror in the high-Q Fabry–Perot resonator. The comparison of the obtained results with earlier works shows that the reflection losses for Nb in the superconducting state is several times lower than those for the high-temperature superconductor YBa2Cu3O7−δ and lower than those for the samples of high-purity copper cooled down to cryogenic temperatures, which is the best of the classical conductors in terms of surface resistance. This allows one to state that niobium and its nitrides cooled down to temperatures below 9 K can be used efficiently as antenna materials (both as coatings on mirror antennas and for manufacture of planar nanoantennas of the detector module) in order to ensure the minimum level of thermal noise in subterahertz radio telescopes with cooled and superconducting receivers.

The results of the development and testing of superconducting integrated microcircuits based on NbTiN/Al transmission lines at the frequencies up to 1.1 THz are presented. The integrated circuits consist of a slot planar antenna made of thin film of NbTiN coupled to a NbTiN/Al microstrip line, and a superconductor-insulator-superconductor (SIS) junction based on Nb/AlN/NbN operating as a terahertz (THz) detector. Two different designs having the operating range of 0.9-1.2 THz were numerically simulated, fabricated and experimentally tested. A strong pumping of SIS detector by a signal of the backward wave oscillator (BWO) around frequency of 1.05 THz was observed, which demonstrated the applicability of fabricated transmission lines at frequencies higher than 750 GHz, where the traditionally used Nb/Nb transmission lines cannot operate.

We present an analysis of a waveguide structure for a 211–275 GHz sideband separating (2SB) mixer based on superconductor–insulator–superconductor (SIS) tunnel junctions. A general analytical model describing the quality of the sideband rejection ratio (SRR) is developed. It shows a crucial influence of reflections from single-ended mixers, reference frequency (RF) load, and the RF hybrid on the SRR level. Due to the intrinsic asymmetry of the 2SB waveguide structure, the reflections strongly affect both the balance of the observed signal and the balance of the local oscillator (LO) pumping signal. The model is verified and confirmed by 3-D electromagnetic simulations showing good qualitative and quantitative agreement. The developed theory gives a practical tool to design 2SB waveguide mixers with a required SRR level. Based on the presented theory, the waveguide structure of the 211–275 GHz 2SB SIS mixer is designed. It is predicted a degradation of the SRR level from 26 dB to about 18 dB due to reflections. The developed model explains some experimental data measured for 2SB SIS mixers developed earlier.

In this article, we perform systematic study of electrodynamic properties of superconducting NbTiN films at frequencies 0.2–2.5 terahertz (THz) and in temperature range from 4 to 15 K using time-domain spectrometer. The goal is to achieve the best parameters of the films at THz frequencies; that is to reach а tradeoff between the highest possible normal state conductivity σ0 , the smallest London penetration depth λL , and the highest energy gap and critical temperature Tc . To do this, it is necessary to determine the optimal manufacturing conditions; to this end, a set of NbTiN films of various compositions was fabricated, controlled by the nitrogen pressure in the magnetron chamber. As a result, the film with parameters σ 0 = 11·10 3 1/(Ohm·сm), λL = 280 nm, and Tc = 14.4 K were obtained. To fit the experimental data, two models with and without taking intragap states into account were used, and both of them are in good agreement with experiment.

A microwave method for measuring the electrical parameters of circuits, including capacitances, using superconducting resonators was proposed and implemented. An original technology of manufacturing compact capacitors in superconducting circuits with a thin anodized insulator layer was used. The structures comprising arrays of such superconducting resonators were fabricated and their spectra were measured. The numerical calculations of the test structures were compared with the spectra measured in the experiment. The values of the capacitance, through which the resonators were coupled to a microwave transmission line, were determined. The obtained values were analyzed, and the reliability of the technique used was assessed.

In this paper, we developed and investigated superconducting integrated circuits designed to match the impedances of a generator represented by a flux-flow oscillator based on a long Josephson junction and a detector based on a superconductor–insulator–superconductor tunnel junction in the subterahertz frequency range. The structures were modeled using the transfer matrix method. Designs were also calculated in the program of numerical three-dimensional modeling. A qualitative agreement was found between the results obtained by two methods. Three samples with different topologies were designed, covering the frequency range of 250–680 GHz at a level of –2 dB.