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We designed, implemented, and characterized differential amplifiers for cryogenic temperatures based on Si bipolar junction transistor technology. The amplifiers show high gain values of more than 60 dB at 300, 77, and 48 K. The minimum voltage noise spectral density was achieved at 77 K and corresponded to 0.33 nV/Hz0.5 with a flicker noise of 20 Hz. The maximum voltage gain was 70 dB at 77 K for a frequency range from DC to 17 kHz. We experimentally show that the parallel differential circuit design allows for a reduction of the voltage noise from 0.55 to 0.33 nV/Hz0.5 at 77 K.

Microwave superconducting quantum circuits are strongly affected by various defects that are unavoidable during the production process. This paper presents a new method for measuring losses in an open transmission line. This line interacts through a capacitive coupling with a coplanar quarter-wavelength resonator, which is used to study losses therein. The authors have investigated the dependencies of losses on power and temperature (in the millikelvin range). It has been shown that major losses in a transmission line are due to the interaction of a microwave field with defects which are effectively described by two-level systems.Microwave superconducting quantum circuits are strongly affected by various defects that are unavoidable during the production process. This paper presents a new method for measuring losses in an open transmission line. This line interacts through a capacitive coupling with a coplanar quarter-wavelength resonator, which is used to study losses therein. The authors have investigated the dependencies of losses on power and temperature (in the millikelvin range). It has been shown that major losses in a transmission line are due to the interaction of a microwave field with defects which are effectively described by two-level systems.

The integration, scale-up, and multiplexing arrays of superconducting qubits in quantum circuits are the main challenges of superconducting quantum technology. Here we experimentally investigate the solid-state qubit multiplexing readout scheme, containing coplanar quarter-wavelength resonators coupled with a planar Xmon-type qubit, connected to a common coplanar transmission line. We find that the qubit energy spectrum is modified in the presence of an additional exciting signal at the fundamental frequency of the neighboring resonators. We attribute the origin of this effect to the electromagnetic field propagating through the common ground plane, which changes the qubit's characteristics. Our finding may be useful for the development of scalable superconducting quantum integrated circuits with arrays of multiplexed or coupled qubits for applications in superconducting quantum processing and computing.

We show an experimental study of superconducting X-mon qubit based on low noise wideband measurement setup. The setup includes wide stop-band low frequency filters with a stopband frequency up to 13 GHz, the broadband low-noise cryogenic microwave amplifier (cLNA) operating at 3.8 K. The obtained power dissipation of the cLNA is below 20 mW, the frequency operating ranges from 6 GHz to 12 GHz with a gain of 30 dB. The equivalent noise temperature of the amplifier is bellow 6 K for the presented frequency range. We demonstrate here the characterization of the superconducting X-mon qubit coupled to an on-chip coplanar waveguide resonator. We show standard qubit experiments: one and two-tones qubit spectroscopy, ac-Stark shift experiments for probing and excitation power sweeps.

A broadband low-noise four-stage high-electron-mobility transistor amplifier was designed and characterized in a cryogen-free dilution refrigerator at the 3.8 K temperature stage. The obtained power dissipation of the amplifier is below 20 mW. In the frequency range from 6 to 12 GHz its gain exceeds 30 dB. The equivalent noise temperature of the amplifier is below 6 K for the presented frequency range. The amplifier is applicable for any type of cryogenic microwave measurements. As an example we demonstrate here the characterization of the superconducting X-mon qubit coupled to an on-chip coplanar waveguide resonator.

The paper deals with silicon-germanium heterojunction transistor’s operation under cryogenic conditions. Evaluating methodology was demonstrated with simple tool set. Families of output and input I-V characteristics in common emitter mode were measured at room and liquid nitrogen temperatures. Some cryogenic effects were observed and discussed, namely: 1) increasing of turn-on voltage due to freezing of major carriers in emitter with cooling, 2) enhancement of current gain due to bandgap difference of Si emitter and SiGe base even at LNT, 3) reducing of breakdown voltage due to more pronounced avalanche multiplication, 4) early voltage decreasing because of neutral base recombination, 5) improvement of power dissipation and reducing of self-heating effect. Despite of these effects, SiGe HBTs save their robust operation at cryogenic temperatures that confirms the capability of used IC technology process for LNT applications.

This paper reports on the results of the investigation of a superconducting flux quantum bit (qubit) in the quasi-dispersive regime, where the frequency of a probe signal even though is lower than, but, nevertheless, close to the excitation frequency of the qubit. In this regime, in contrast to the known experiments, the interaction of the qubit with a waveguide leads not only to a shift of the resonance frequency, which is characteristic of the dispersive regime, but also to a significant broadening of the resonance line due to the spontaneous emission of the qubit. On the basis of the analysis of the amplitude–frequency characteristic of the transmission signal, this makes it possible to determine, under single-frequency excitation, the characteristic parameters of the qubit inductively coupled to the coplanar resonator.

A low-noise cryogenic amplifier for the measurement of weak microwave signals at sub-Kelvin temperatures is constructed. The amplifier has five stages based on SiGe bipolar heterostructure transistors and has a gain factor of 35 dB in the frequency band from 100 MHz to 4 GHz at an operating temperature of 800 mK. The parameters of a superconducting quantum bit measured with this amplifier in the ultralow-power mode are presented as an application example. The amplitude–frequency response of the “supercon-ducting qubit–coplanar cavity” structure is demonstrated. The ground state of the qubit is characterized in the quasi-dispersive measurement mode.

The properties of a superconducting flux quantum bit (qubit) in the quasidispersive mode, where the frequency of a probe signal is lower than the qubit excitation frequency but is close to it, have been experimentally studied. It has been shown that all parameters of the qubit inductively coupled to a coplanar resonator can be determined at the single-frequency excitation from the analysis of the frequency responses of the transmission of the probe signal at the output of this resonator. Under the additional excitation of the qubit by the signal at the second harmonic of the cavity, resonance dips have been observed because of resonance between the probe signal and induced Rabi splitting. It has been shown that the positions of these dips are in good agreement with the parameters of the qubit that are obtained by analyzing the amplitude–frequency response within the width of the fundamental resonance frequency.

This work deals with investigation of CMOS operation at cryogenic conditions. Logic inverter made using 250 nm BCD technology process has been examined at 300, 77 and 48 K. Transfer and output static characteristics of CMOS inverter and characteristics of its transistors have been obtained and analyzed.

Applications of up-to-date cryoelectronics, especially in quantum microwave engineering, require instruments that operating identically to an optical beam splitter. Using niobium technology for the application of thin film structures, we have fabricated a microwave signal power splitter with central frequency f0 = 7.2 GHz. We have demonstrated experimentally that, in the 1-GHz band around central frequency f0, the characteristics of the second and the third output ports are in the range of −3 ± 0.8 dB of the signal input power. The attenuation within power transfer from signal ports to the insulation port is 10 dB. The above-mentioned characteristics have been obtained at an operating temperature below the superconducting transition temperature.

We report measurements of transfer functions and flux shifts of 20 on-chip high T$_C$ DC SQUIDs half of which were made purposely geometrically asymmetric. All of these SQUIDs were fabricated using standard high T$_C$ thin film technology and they were single layer ones, having 140 nm thickness of YBa$_2$Cu$_3$O$_{7-x}$ film deposited by laser ablation onto MgO bicrystal substrates with 24$^0$ misorientation angle. For every SQUID the parameters of its intrinsic asymmetry, i. e., the density of critical current and resistivity of every junction, were measured directly and independently. We showed that the main reason for the on-chip spreading of SQUIDs' voltage-current and voltage-flux characteristics was the intrinsic asymmetry. We found that for SQUIDs with a relative large inductance ($L>120 $ pH) both the voltage modulation and the transfer function were not very sensitive to the junctions asymmetry, whereas SQUIDs with smaller inductance ($L\simeq 65-75 $ pH) were more sensitive. The results obtained in the paper are important for the implementation in the sensitive instruments based on high T$_C$ SQUID arrays and gratings., 11 pages, 4 tables, 17 figures This version is substantially modified. The Introduction and Section 2 are completely rewritten, while experimental part is mainly the same as in previous version

A theory for the voltage-current characteristic in high TC DC SQUIDs, which accounts for a second harmonic in the junction current-phase relation, is developed. The comparison with experiment is performed. It is shown that if the second harmonic is prevailed, the theory can explain the large deviations of the experimental voltage modulation from theoretical predictions and computer simulations based on conventional sinusoidal current-phase relation., Comment: Eq. 13 is corrected

The complex impedance measurement technique is an established method for the characterisation of Transition-Edge Sensors (TES) bolometers and microcalorimeters. In this report this technique is applied at frequencies up to 250kHz to Ti/Pd and Ti/Au Transition-Edge Sensors, in the optical and near-infrared wavelength range, to obtain a complete frequency response in the complex plane. From these measurements we obtain information about the logarithmic temperature sensitivity α, the logarithmic current sensitivity β and on the device thermal parameters, heat capacity C, thermal conductance G and the effective response time τ eff. All these parameters provide a reasonable understanding of the two different detectors and a discussion about the main differences is also reported.

The analytical theory for the voltage-current characteristics of the large inductance (L>100 pH) high-T_c DC SQUIDs that has been developed previously is consistently compared with the computer simulations and the experiment. The theoretical voltage modulation for symmetric junctions is shown to be in a good agreement with the results of known computer simulations. It is shown that the asymmetry of the junctions results in the increase of the voltage modulation if the critical current is in excess of some threshold value (about 8 microAmps). Below this value the asymmetry leads to the reduction of the voltage modulation as compared to the symmetric case. The comparison with the experiment shows that the asymmetry can explain a large portion of experimental values of the voltage modulation which lie above the theoretical curve for symmetric DC SQUID. It also explains experimental points which lie below the curve at small critical currents. However, a significant portion of these values which lie below the curve cannot be explained by the junction asymmetry., 22 pages, 14 figures

A high temperature superconductor (HTS) SQUID photoscanning system for nondestructive evaluation of photovoltaic devices has been developed. The SQUID is cooled to its operational temperature of 77 K by a commercial Stirling-type cryocooler with a maximum cooling power of 3W at 80 K. The system can be operated completely unshielded. For this type of system, an HTS washer-type dc SQUID with a small sensing area of about 500 /spl mu/m/spl times/500 /spl mu/m has been fabricated. To stabilize the working point of the SQUID in the cryocooler based system a new direct-coupled (dc) read-out electronics with automatic bias voltage tuning has been used. The samples analyzed are multicrystalline silicon wafers with lateral p-n junctions and complete solar cells fabricated on such substrates. The HTS SQUID photoscanning system has a field noise level of 7 pT/H/sup 1/2/ at 1 kHz, the modulation frequency of the exciting laser beam. A typical signal-to-noise ratio (SNR) of 50 has been achieved for solar cell analysis. It has been be demonstrated that typical defects and structural features of solar cells and multicrystalline substrates can be characterized.

Impedance measurements of superconducting transition-edge sensors (TESs) are a powerful tool to obtain information about the TES thermal and electrical properties. We apply this technique to a 20??m ? 20??m Ti/Au TES, suitable for application in the optical and near-infrared range, and extend the measurements up to 250?kHz in order to obtain a complete frequency response in the complex plane. From these measurements we obtain important thermal and electrical device parameters such as heat capacity C, thermal conductance G and effective thermal time constant ?eff that will be compared with the corresponding values obtained from noise measurements.

Recently, we have developed a 5-channel high-Tc SQUID system with one signal channel intended for rat magnetocardiography (MCG) in moderately shielded or "quiet" real environment. This system is an adapted version of a human MCG system which has been improved with respect to user-friendliness and stability. A dewar with a cold-warm distance of 7 mm and a refill cycle time of up to one week is utilized. The implemented high-Tc SQUIDs are single-layer devices with grain boundary junctions fabricated at KRISS with laser ablation on 10 mm × 10 mm STO substrates. In order to cancel environmental magnetic noise, three of the five SQUIDs are arranged to build an axial software first-order or second-order gradiometer with a base line of 35 mm. The other two SQUIDs are used for balancing. To overcome previous system instabilities, we have implemented an Earth field compensation for each SQUID. For this, the SQUIDs were mounted in capsules containing integrated field compensation coils. The three Earth field components are measured with an additional triaxial fluxgate, and compensated at the SQUID locations using the low-noise current source of the SQUID readout electronics. This way, the SQUIDs can be cooled and operated in a low residual field that improves system stability and reduces low-frequency SQUID noise. It is even possible to slowly move the dewar in the Earth field (dynamic field compensation). Different noise cancellation procedures were optimized and compared employing a periodic signal source.