Normal Metal Insulator Superconductor Research Articles

Propagation velocity measurements of substrate phonon bursts using MKIDs for superconducting circuits

High-energy bursts in superconducting quantum circuits from various radiation sources have recently become a practical concern due to induced errors and their propagation in the chip. The speed and distance of these disturbances have practical implications. We used a linear array of multiplexed MKIDs on a single silicon chip to measure the propagation velocity of a localized high-energy burst, introduced by driving a normal metal-insulator-superconductor (NIS) junction. We observed a reduction in the apparent propagation velocity with NIS power, which is due to the combined effect of reduced phonon flux with distance and the existence of a minimum detectable QP density in the MKIDs. A simple theoretical model is fitted to extract the longitudinal phonon velocity in the substrate and the conversion efficiency of phonons to QPs in the superconductor.

Structure of aluminum films for creating tunnel junctions.

A series of studies of the structure of aluminum films deposited on single-crystalsilicon substrates in different temperature conditions. The roughness and grain size of films of nuclei 20 nm thick, deposited at elevated temperatures, and also dusted on top of the seed layer at room temperature to a thickness of 150 nm. The film profile was measured in an electron microscope. It was found that films on the hot sublayer turn out to be smoother, more rigid (less loose) and allow one to expect the creation of superconductor–insulator–superconductor and superconductor–insulator–normal metal transitions, respectively, with a higher current density and lower capacitance.

Application of the pseudo-Voigt function to the analysis of single-particle tunneling spectra of [formula omitted]-wave superconductors

In this study, we focused on the peak–dip–hump structure, which is a hallmark of cuprate superconductivity and is often used to explore the relationship between strong electron–phonon coupling and superconductivity. Specifically, we examined the peak–dip–hump structure and other features of cuprate tunneling spectra in d-wave superconductor–insulator–normal metal (dSC–I–N) junctions. To demonstrate the peak–dip–hump structure and other features of cuprate tunneling spectra, we applied the pseudo-Voigt function as an energy-dependent gap. We also analyzed two important experimental features of the tunneling spectra, namely the temperature and doping evolution of the differential conductance. Finally, we compared our calculation results with experimental data and found a good quantitative agreement between theory and experimental data in the considered cases.

The inverse proximity effect in tunnel heterostructure ferromagnetic–insulator–superconductor Co2CrAl–I–Pb

Ferromagnetic–insulator–superconductor Co2CrAl–I–Pb tunnel junctions were created and their current-voltage characteristics (CVCs) were studied. It has been established that they differ from the usual CVCs of tunnel junctions of a normal metal–insulator–superconductor. It is proposed to assume that the observed effect of changing the type of CVC is associated with the existence of the destruction of Cooper pairs due to the inverse effect of proximity between the Co2CrAl and Pb electrodes. The CVCs were calculated based on the approach of P. Fulde.

Electrical and thermal transport through NIS junction

We investigate the electrical and thermal transport properties of the based normal metal–insulator–superconductor (NIS) junction using Blonder–Tinkham–Klapwijk theory. We show that the tunneling conductance of the NIS junction is an oscillatory function of the effective barrier potential (χ) of the insulating region up to a thin barrier limit. The periodicity and the amplitudes of the oscillations largely depend on the values of α and the gate voltage of the superconducting region, namely, U 0. Further, the periodicity of the oscillation changes from π to as we increase U 0. To assess the thermoelectric performance of such a junction, we have computed the Seebeck coefficient, the thermoelectric figure of merit, maximum power output, efficiency at the maximum output power of the system, and the thermoelectric cooling of the NIS junction as a self-cooling device. Our results on the thermoelectric cooling indicate practical realizability and usefulness for using our system as efficient cooling detectors, sensors, etc and hence could be crucial to the experimental success of the thermoelectric applications of such junction devices. Furthermore, for an lattice, whose limiting cases denote a graphene or a dice lattice, it is interesting to ascertain which one is more suitable as a thermoelectric device and the answer seems to depend on the U 0. We observe that for an lattice corresponding to , graphene (α = 0) is more feasible for constructing a thermoelectric device, whereas for , the dice lattice (α = 1) has a larger utility.

Initial experimental results on a superconducting-qubit reset based on photon-assisted quasiparticle tunneling

We present here our recent results on qubit reset scheme based on a quantum-circuit refrigerator (QCR). In particular, we use the photon-assisted quasiparticle tunneling through a superconductor–insulator–normal-metal–insulator–superconductor junction to controllably decrease the energy relaxation time of the qubit during the QCR operation. In our experiment, we use a transmon qubit with dispersive readout. The QCR is capacitively coupled to the qubit through its normal-metal island. We employ rapid, square-shaped QCR control voltage pulses with durations in the range of 2–350 ns and a variety of amplitudes to optimize the reset time and fidelity. Consequently, we reach a qubit ground-state probability of roughly 97% with 80-ns pulses starting from the first excited state. The qubit state probability is extracted from averaged readout signal, where the calibration is based on Rabi oscillations, thus not distinguishing the residual thermal population of the qubit.

Microwave SINIS Detectors

This review presents the main characteristics and mechanisms of operation of superconductor–insulator–normal metal–insulator–superconductor (SINIS) microwave detectors. An analysis of the detectors’ performance against a quantum detector and a photon counter is given. Methods for cooling a superconductor using normal metal traps and the role of electron cooling in optimizing the current response to terahertz radiation are discussed. Fabrication methods using shadow evaporation as well as magnetron sputtering are described.

Fast Variable-Temperature Cryogenic Blackbody Sources for Calibration of THz Superconducting Receivers

An electrically heated blackbody radiation source comprising thin metal film on a dielectric substrate and an integrating cavity was designed, fabricated, and experimentally studied at frequencies from 75 to 500 GHz. Analytical and numerical modeling were performed to optimize the emissivity, spectral uniformity, and modulation frequency of the radiation source with the spherical integrating cavity and thin film absorber. The blackbody emissivity (absorptivity) increased from 0.3 to 0.5 for the bare thin film on dielectric substrate, and up to 0.95 when it was placed inside the integrating cavity. The fabricated source mounted at the 0.5 K stage was used to measure the response time of a few microseconds and for sensitivity measurement down to 10−18 W/Hz1/2 of the superconductor–insulator–normal metal–insulator–superconductor (SINIS) detector at 100 mK.

Single-junction quantum-circuit refrigerator

We propose a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single normal-metal–insulator–superconductor (NIS) junction. In contrast to previous studies with multiple junctions and an additional charge island for the QCR, we directly connect the NIS junction to an inductively shunted electrode of a superconducting microwave resonator making the device immune to low-frequency charge noise. At low characteristic impedance of the resonator and parameters relevant to a recent experiment, we observe that a semiclassical impedance model of the NIS junction reproduces the bias voltage dependence of the QCR-induced damping rate and frequency shift. For high characteristic impedances, we derive a Born–Markov master equation and use it to observe significant non-linearities in the QCR-induced dissipation and frequency shift. We further demonstrate that, in this regime, the QCR can be used to initialize the linear resonator into a non-thermal state even in the absence of any microwave drive.

Josephson Tunnel Junctions with an Integral Superconductor–Insulator–Normal Metal Shunt

Josephson Tunnel Junctions with an Integral Superconductor–Insulator–Normal Metal Shunt

Fabrication of NIS and SIS Nanojunctions with Aluminum Electrodes and Studies of Magnetic Field Influence on IV Curves

Samples of superconductor–insulator–superconductor (SIS) and normal metal–insulator–superconductor (NIS) junctions with superconducting aluminum of different thickness were fabricated and experimentally studied, starting from conventional shadow evaporation with a suspended resist bridge. We also developed alternative fabrication by magnetron sputtering with two-step direct e-beam patterning. We compared Al film grain size, surface roughness, resistivity deposited by thermal evaporation and magnetron sputtering. The best-quality NIS junctions with large superconducting electrodes approached a resistance R(0)/R(V2Δ) factor ratio of 1000 at 0.3 K and over 10,000 at 0.1 K. At 0.1 K, R(0) was determined completely by the Andreev current. The contribution of the single-electron current dominated at V > VΔ/2. The single-electron resistance extrapolated to V = 0 exceeded the resistance R(V2Δ) by 3 × 109. We measured the influence of the magnetic field on NIS junctions and described the mechanism of additional conductivity due to induced Abrikosov vortices. The modified shape of the SINIS bolometer IV curve was explained by Joule overheating via NIN (normal metal–insulator–normal metal) channels.

Charge dynamics in quantum-circuit refrigeration: Thermalization and microwave gain

Previous studies of photon-assisted tunneling through normal-metal–insulator–superconductor junctions have exhibited potential for providing a convenient tool to control the dissipation of quantum-electric circuits in situ. However, the current literature on such a quantum-circuit refrigerator (QCR) does not present a detailed description for the charge dynamics of the tunneling processes or the phase coherence of the open quantum system. Here, we derive a master equation describing both quantum-electric and charge degrees of freedom, and discover that typical experimental parameters of low temperature and yet lower charging energy yield a separation of time scales for the charge and quantum dynamics. Consequently, the minor effect of the different charge states can be taken into account by averaging over the charge distribution. We also consider applying an ac voltage to the tunnel junction, which enables control of the decay rate of a superconducting qubit over four orders of magnitude by changing the drive amplitude; we find an order-of-magnitude drop in the qubit excitation in 40 ns and a residual reset infidelity below 10−4. Furthermore, for the normal island, we consider the case of charging energy and single-particle level spacing large compared to the superconducting gap, i.e., a quantum dot. Although the decay rates arising from such a dot QCR appear low for use in qubit reset, the device can provide effective negative damping (gain) to the coupled microwave resonator. The Fano factor of such a millikelvin microwave source may be smaller than unity, with the latter value being reached close to the maximum attainable power.

Fast unconditional initialization for superconducting qubit and resonator using quantum-circuit refrigerator

In a quantum computation scheme, such as recycling a qubit, the initialization of the qubit is always required. At present, the initialization of the qubit is one of the bottlenecks in such computation, and high fidelity and fast initialization still remain important research subjects. In this study, by coupling a SINIS (superconductor–insulator–normal metal–insulator–superconductor) junction to a coupled system consisting of a qubit and a resonator, a photon is removed from the resonator by single-photon-assisted tunneling, and the effective relaxation rate of the resonator is increased. In addition, by applying an |e,0⟩−|f,0⟩ drive pulse and an |f,0⟩−|g,1⟩ drive pulse to the qubit, the energy of the qubit is rapidly transferred to the resonator to realize high-speed initialization. The present simulation shows that the qubit can be initialized with 99% fidelity in 80 ns and 99.9% in 137 ns when the electron temperature of the normal metal of the SINIS junction is 10 mK. The simulation also shows that the relaxation time of the qubit is about 13% longer when the resonator is interposed between the SINIS junction and the qubit than when the SINIS junction is directly coupled to the qubit.

Superconducting tunnel junction fabrication on three-dimensional topography based on direct laser writing

Superconducting junctions are widely used in a multitude of applications ranging from quantum information science and sensing to solid-state cooling. Traditionally, such devices must be fabricated on flat substrates using standard lithographic techniques. In this study, we demonstrate a highly versatile method that allows for superconducting junctions to be fabricated on a more complex topography. It is based on maskless direct laser writing and two-photon lithography, which allows writing in 3D space. We show that high-quality normal metal–insulator–superconductor tunnel junctions can be fabricated on top of a 20-μm-tall three-dimensional topography. Combined with conformal resist coating methods, this technique could allow sub-micron device fabrication on almost any type of topography in the future.

Superconducting Receivers for Space, Balloon, and Ground-Based Sub-Terahertz Radio Telescopes

We give a review of both our own original scientific results of the development of superconducting receivers for sub-terahertz astronomy and the main leading concepts of the global instrumentation. The analysis of current astronomical problems, the results of microwave astroclimate research, and the development of equipment for sub-terahertz radio astronomy studies justify the need and feasibility of a major infrastructure project in Russia to create a sub-terahertz telescope, as well as to enhance the implementation of the ongoing Millimetron and Suffa projects. The following results are discussed: i) superconducting coherent receivers and broadband subterahertz detectors for space, balloon, and ground-based radio telescopes have been developed and tested; ii) ultrasensitive receiving systems based on tunnel structures such as superconductor—insulator—superconductor (SIS) and superconductor—insulator—normal metal—insulator—superconductor (SINIS) have been created, fabricated, and examined; iii) a receiving array based on SINIS detectors and microwave readout system for such structures has been implemented; iv) methods for manufacturing high-quality tunnel structures Nb/AlOx/Nb and Nb/AlN/NbN based on niobium films with a current density of up to 30 kA/cm2 have been developed. Receivers operated at 200 to 950 GHz and having a noise temperature only a factor of 2 to 5 higher than the quantum limit have been created and tested.

SINIS Bolometer with Microwave Readout

A bolometer, which is based on a superconductor–insulator–normal metal–insulator–superconductor (SINIS) structure, integrated into a twin-slot antenna with a central frequency of 90 GHz, and connected with a superconducting microwave readout resonator, has been designed, fabricated, and experimentally studied. Such an elementary cell is designed for the multi-element array of a high-sensitive radioastronomic receiver, in which the readout from a great number of channels is performed by a single coaxial cable instead of separate wires and amplifier in each channel.

Quantum Response of a Bolometer Based on the SINIS Structure with a Suspended Absorber

Bolometers based on the superconductor–insulator–normal-metal–insulator–superconductor (SINIS) structure with an absorber suspended above the substrate are developed, fabricated, and experimentally studied in the terahertz frequency range. In contrast to the previously studied bolometers with an absorber located directly on the substrate, the real bolometric mode of operation is achieved, i.e., there is more than one excited electron per quantum of electromagnetic radiation (the quantum efficiency is larger than 1). A quantum efficiency of fifteen electrons per quantum of radiation with a frequency of 350 GHz was achieved in the studied bolometers.

Matching of Radiation with Array of Planar Antennas with SINIS Bolometers in an Integrating Cavity

Abstract—Arrays of planar annular antennas in the 345 GHz range with integrated superconductor–insulator–normal metal–insulator–superconductor (SINIS) bolometers have been developed, fabricated, and experimentally investigated. To increase the absorption efficiency, the source signal was matched with the array using a back-to-back horn and a counter-reflector. The efficiency of the receiving structure was studied depending on the direction of irradiation—through the substrate and from the antenna—as well as on the thickness of the substrate. The methods for normalizing a received signal using a reference channel outside and inside a cryostat is described. The best results were obtained by irradiating the array from the antenna with a substrate thickness equivalent to a quarter wavelength in the dielectric and sputtering of the rear side with a gold film (counterreflector). The effective matching band in the main range was more than 50 GHz; the volt–watt sensitivity reaches 109 V/W.

Calibration of cryogenic amplification chains using normal-metal–insulator–superconductor junctions

Various applications of quantum devices call for an accurate calibration of cryogenic amplification chains. To this end, we present an experimentally feasible calibration scheme and use it to accurately measure the total gain and noise temperature of an amplification chain by employing normal-metal–insulator–superconductor (NIS) junctions. Our method is based on the radiation emitted by inelastic electron tunneling across voltage-biased NIS junctions. We derive an analytical expression that relates the generated power to the applied bias voltage which is the only control parameter of the device. After the setup has been characterized using a standard voltage reflection measurement, the total gain and the noise temperature are extracted by fitting the analytical expression to the microwave power measured at the output of the amplification chain. The 1σ uncertainty of the total gain of 51.84 dB appears to be of the order of 0.10 dB.

Investigation of the Speed of a SINIS Bolometer at a Frequency of 350 GHz

We have measured the time of the optical response of a 350-GHz radiation detector based on a superconductor–insulator–normal metal–insulator–superconductor tunnel structure with a suspended normal-metal bridge integrated into a planar log-periodic antenna. The transient characteristics of the detector were recorded when irradiated by a fast cryogenic blackbody source with a rise time of the order of microseconds. For this purpose, a short intense heating pulse was fed to an emitting NiCr film radiation source with a low heat capacity. The measured response time was 1.8 ± 0.5 μs at a bolometer electron temperature of 0.17 K, with a detection sensitivity of 10–17–10–18 W Hz–1/2 being potentially achievable.