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Waveguides with superconducting Josephson junction-based metamaterial are widely used as parametric amplifiers. However, the precise estimation of power entering the device is crucial for the estimation of gain and noise temperature. This is nontrivial when the measurement tract is not symmetrical. We present a basic framework for the analysis of properties of such nonlinear systems and calibration of the input power. Utilizing measurements with varied temperature and power of the input signal, we estimate additional attenuation of the input line. We demonstrate a precise calibration procedure of a Josephson junction metamaterial.

Current-biased Josephson junctions are promising candidates for the detection of single photons in the microwave frequency domain. With modern fabrication technologies, the switching properties of the junction can be adjusted to achieve quantum limited sensitivity. Namely, the width of the switching current distribution can be reduced well below the current amplitude produced by a single photon trapped inside a superconducting cavity. However, for an effective detection a strong junction cavity coupling is required, providing nonlinear system dynamics. We compare experimental findings for our prototype device with a theoretical analysis aimed to describe the switching dynamics of junctions under microwave irradiation. Measurements are found in qualitative agreement with our simulations.

The Josephson junction as a switching detector of weak signals is investigated in presence of noise in the frame of rotating pendulum model. The parameter range, where the detection can be more efficient, is found. It has been demonstrated, that with decrease of the signal power the double minima of the mean switching time and the standard deviation are transformed into a single minimum, which corresponds to interplay between noise suppression and resonant activation regimes. The resonant nature of escape allows to detect weak signals, whose amplitudes are weaker than the difference between critical current and bias current of a Josephson junction. With decrease of damping an efficient detection becomes possible even at subharmonics of the resonance frequency.

Technological processes for the fabrication of low- and high- Tc Josephson junctions, aimed for certain applications, are described. On the one hand, the integration of low- Tc superconductor digital electronics with superconducting sensor arrays enables input signal processing with quantum limited resolution at millikelvin temperatures. We describe this mixed signal superconductor technology for analogue sensor readout and signal multiplexing for operating temperatures down to 300 mK. On the other hand, by making use of modern high- Tc Josephson junction technology, sensitive magnetometers, which require a modest cooling power, can be developed. Examples of the application of the mentioned processes are shown.

The possibility of the implementation of a single-photon detector in the microwave range has been discussed. It has been shown that, for these purposes, it is possible to use an unshunted Josephson junction, which is switched from the superconducting state into a finite-voltage state at the presence of an external signal. The sensitivity of this detector is determined by the distribution of switchings in the absence of an external signal. It has been demonstrated that there is a Josephson junction with a noise temperature below 60 mK at a nominal external temperature of 10 mK. A specific detector intended for the measurement of external microwave signals has been proposed, designed, and fabricated. The first experimental results have been presented.

We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a {\lambda}/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations., Comment: 5 pages, 5 figures

We present two possible methods for the fabrication of sub-micron sized Josephson junctions, namely the shadow-evaporation technique and the cross-type technology. Their importance for the field of modern super-conducting technology is discussed. As examples we present measurement results of a two-qubit sample and a prototype of a microwave detector fabricated each by one of the described methods. We review potential applications of superconducting quantum circuits based on the developed methods.

We demonstrate amplification (and attenuation) of a probe signal by a driven two-level quantum system in the Landau-Zener-St\"uckelberg-Majorana regime by means of an experiment, in which a superconducting qubit was strongly coupled to a microwave cavity, in a conventional arrangement of circuit quantum electrodynamics. Two different types of flux qubit, specifically a conventional Josephson junctions qubit and a phase-slip qubit, show similar results, namely, lasing at the working points where amplification takes place. The experimental data are explained by the interaction of the probe signal with Rabi-like oscillations. The latter are created by constructive interference of Landau-Zener-St\"uckelberg-Majorana (LZSM) transitions during the driving period of the qubit. A detailed description of the occurrence of these oscillations and a comparison of obtained data with both analytic and numerical calculations are given.

The Josephson effect in ballistic point contacts between single-band and multi-band superconductors was investigated. It was found that in the case of Josephson junctions formed by a single-band and an $s_\pm$-wave two-band superconductor as well as by a single-band and a three-band superconductor the junctions become frustrated, demonstrating the $\phi$-contact properties. Depending on the ground state of a three-band superconductor with broken time-reversal symmetry (BTRS), the Josephson junction can have from one to three energy minima, some of which can be locally stable. We also study the behavior of a dc SQUID based on the Josephson junctions between single-band and multi-band superconductors. Some features on the dependences of the critical current and the total magnetic flux on the applied flux of a dc SQUID based on the Josephson point contacts between a single-band superconductor and an $s_\pm$-wave superconductor, three-band superconductor with BTRS and three-band superconductor without BTRS as compared to the conventional dc SQUIDs based on single-band superconductors were found. The results can be used as an experimental tool to detect the existence of multi-band structure and BTRS., Comment: 23 pages, 11 figures

The effect of an array of ferromagnetic nanoparticles on the field-dependent critical current of the short overlap Josephson junction is experimentally studied. Large reversible variations of the maximum critical current are observed depending on the magnetic state of the particles. The pronounced commensurability effects are detected which are proved by the additional peaks of magnetic field induced diffraction pattern.

We study theoretically dynamic properties of an interferometer-type Josephson charge qubit consisting of a single-Cooper-pair transistor (with capacitive gate) closed by a superconducting inductive loop. Analyzing the radio frequency response of the device to microwave fields within a two-band model, we present basic features of a method for observation of various Rabi oscillation modes. For this purpose, we apply the dressed-state approach in conjunction with the rotating-wave approximation to the strongly driven quantum bit. We demonstrate the existence of multiphoton excitations and Rabi frequency transitions between quantum levels connected with them. The resulting oscillations of the supercurrent in the interferometer modify the spectral loop susceptibility. This effect may be detected via the coil of a high-quality tank circuit inductively coupled to the qubit by measuring the tank-qubit impedance variations belonging to it.

The effect of a system of ferromagnetic particles on the field-dependent critical current of a Josephson junction is experimentally studied for junctions of different geometries. For edge junctions, the effect of commensurability between the periodic magnetic field of the particles and the Josephson vortex lattice is observed. The effect manifests itself in additional maxima of the field-dependent critical current. For overlap junctions, giant (greater than sixfold) variations of the maximum critical current are observed depending on the magnetic state of the particles. The changes in the “Fraunhofer” pattern of the overlaped Josephson junctions are attributed to the formation of Abrikosov vortices due to the effect of uniformly magnetized particles. The effects revealed in the experiments can be used to analyze the inhomogeneous magnetic field of a system of submicron particles and to control the transport properties of Josephson junctions.

This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions. The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form. A new experimental tool for measuring the CPhiR is described and its practical applications are discussed. The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy. A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications.

We have studied dynamic properties of a special Josephson charge qubit. The device consists of a single-Cooper-pair transistor (with capacitive gate) closed by a superconducting inductive loop. The phase bias is implemented by an external flux that threads the loop. Restricting ourselves to a two-band model, we have derived analytical expressions for the oppositely circulating ring currents in terms of gate charge and phase bias. They permit the determination of important device characteristics such as critical current, loop parameters, and Josephson inductance. For the purpose of measurements in connection with macroscopic quantum coherence, we investigate the qubit's microwave response within the frame of Bloch equations with phenomenological relaxation rates. We find the (nonlinear) qubit impedance in terms of quality, detuning, and the quantum resistance unit, which reflects the competition between irradiation and dephasing.

c-axis oriented and 11/spl deg/ tilted YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) thin films were deposited on [110] NdGaO/sub 3/ and (7 10 2) NdGaO/sub 3/ substrates correspondingly. Nb/Au/YBCO heterojunctions were fabricated using photolithography and ion beam milling. I-V curves of the heterojunctions based on tilted YBCO films showed zero bias anomaly conductance peak and large excess current. We observed the second harmonic (/spl prop/ sin 2/spl phi/) of superconducting current in Nb/Au/YBCO heterojunctions in c-axis oriented YBCO films, although sinusoidal superconducting current-phase relation has been measured for heterojunctions in tilted ones. The obtained results are explained by d/spl plusmn/s symmetry of YBCO superconducting order parameter.

Results of the recent experiments have clear and convincingly demonstrated that superconducting circuits with certain parameters can be in quantum superposition of macroscopically distinct states. This has important implications, for instance, for the quantum measurement theory, for the description of the interaction between light and matter as well as for implementation of new generation of quantum limited detectors and quantum information processing devices. Here both – basic ideas and practical realization of superconducting qubits – are discussed.

Using an RF-SQUID configuration we have measured the temperature dependence of the current–phase relation (CPR) of Nb/InAs(2DEG)/Nb (two-dimensional electron gas, 2DEG) Josephson junctions with high interface transparency. At low temperatures, significant deviations from a sinusoidal CPR are observed. The experimental results are fitted by the scattering matrix theory of Brouwer and Beenakker, additionally taking into account the proximity effect between the Nb and the 2DEG in the inversion layer on p-type bulk InAs. We have studied also the influence of an external gate voltage applied to the InAs(2DEG) channel. It is shown that the amplitude of RF-SQUID oscillations can be controlled by gate voltage with only modest deterioration of the SQUID signal.

We have shown by means of measurements of the current–phase relationship I ( φ ) of YBCO grain boundary Josephson junctions (GBJJs) that GBJJs with misorientation angles θ ⩾24° are in the tunnel limit. The properties of 45° GBJJs are known to be anomalous: both symmetric and asymmetric junctions exhibit a substantial second harmonic of I ( φ ) and, in symmetric 45° GBJJs, a temperature-controlled sign change of the first harmonic of I ( φ ) has been observed. We have shown that in the tunnel limit these anomalous properties follow from the d-wave pairing symmetry of YBCO, provided the interface is sufficiently rough.

The superconducting current–phase relation (CPR) of Nb/Au/(0 0 1) YBa2Cu3Ox (YBCO) heterojunctions, prepared on epitaxial c-axis oriented YBCO thin films, has been measured in a single-junction interferometer. For the first time, the second harmonic of the CPR of such junctions has been observed. The appearance of the second harmonic and the relative sign of the first and second harmonics of the CPR can be explained assuming, that the macroscopic pairing symmetry of our YBCO thin films is of the d+s type.

We have measured the current-phase relationship $I(\ensuremath{\phi})$ of symmetric 45\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ grain boundary Josephson junctions. Substantial deviations of the Josephson current from conventional tunnel-junction behavior have been observed: (i) The critical current exhibits, as a function of temperature $T$, a local minimum at a temperature ${T}^{*}$. (ii) At $T\ensuremath{\approx}{T}^{*}$, the first harmonic of $I(\ensuremath{\phi})$ changes sign. (iii) For $Tl{T}^{*}$, the second harmonic of $I(\ensuremath{\phi})$ is comparable to the first harmonic, and (iv) the ground state of the junction becomes degenerate. The results are in good agreement with a microscopic model of Josephson junctions between $d$-wave superconductors.

We report an experimental and theoretical study of various metastable states of a superconducting loop including a Josephson junction (a single-junction superconducting quantum interferometer). In the experiment, the metastable states manifest themselves by sharp drops in the magnetic-field dependence of the interferometer's response to an externally applied weak alternating bias current. At low temperatures we also observe satellite drops that appear at small values of ac bias. By making use of a mapping to the theory of nonlinear oscillator, we argue that this feature is due to the presence of nonequilibrium ac-induced libration states of the interferometer.

The signal properties of radio frequency (rf) SQUIDs with a finite amplitude of the second harmonic in the current-phase relationship have been theoretically and experimentally investigated. Both amplitude as well as phase detection has been considered in order to readout the SQUID's output signal. We observed peculiarities of the rf SQUID response of a SQUID with YBCO 45 degree grain boundary Josephson junction. We found that the current-phase relationship contains a finite amplitude of the second harmonic. Comparison of the theoretical and experimental results is given. It is shown that the sign of the amplitude of the second harmonic can be determined from the shape of experimental curves, and the value of this amplitude is defined by the positions of extrema of these curves.

The supercurrent-phase relation (CPR) of NbAl based Josephson elements of superconductor/insulator/normal conductor/insulator/superconductor-type is deduced from impedance measurements of the phase-biased junction. The chosen temperature range from 1.9 to 8.0 K covers the superconducting transition of the aluminum interlayer below which a pronounced non-harmonic CPR is found. At temperatures higher than the corresponding critical temperature, the CPR did not deviate significantly from purely harmonic shape. Both results are in agreement with theoretical predictions.

We report the parametric amplification of a microwave signal in a Kerr medium formed from superconducting qubits. Two mutually coupled flux qubits, embedded in the current antinode of a superconducting coplanar waveguide resonator, are used as a nonlinear element. Shared Josephson junctions provide the qubit-resonator coupling, resulting in a device with a measured gain of about 20 dB. We argue, that this arrangement represents a unit cell which can be straightforwardly extended to a quasi one-dimensional quantum metamaterial with a large tunable Kerr nonlinearity., 4 pages, 4 figures

We experimentally investigate the current-phase relation (CPR) of an ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/{\mathrm{A}\mathrm{l}/\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}$ Josephson junction when it is cooled down to the superconducting transition of the aluminum interlayer at about 1.9 K. While the critical current rises by more than one order of magnitude, the CPR changes from sinusoidal to nonsinusoidal behavior. The experimental results are compared with the predictions of a recently developed microscopic model for double-barrier junctions in the dirty limit. We have found quantitative agreement between experiment and theory regarding both, the rise of critical current and the shape of the CPR.

Single-layer washer-type high- T c YBa 2 Cu 3 O 7− x rf SQUIDs with grain-boundary Josephson junctions, as well as low- T c Nb rf SQUIDs with Nb–Al 2 O 3 –Nb tunnel junctions, have been investigated in finite magnetic fields. It was shown experimentally that the suppression of the critical current of the Josephson junction due to the magnetic field leads to a modulation of the amplitude of the SQUID output signal. The role of the “unwanted” junction in high- T c rf SQUIDs, which is formed by the grain boundary running through the washer of the SQUIDs on bicrystal substrates, has also been clarified. The drop of the SQUID signal at a finite magnetic field is originated by the penetration of the magnetic field into the unwanted junction. Based on these results, a direct radio-frequency method for the determination of the first critical field H c1 for long Josephson junctions has been developed.

The current-phase relation (CPR) for asymmetric 45\ifmmode^\circ\else\textdegree\fi{} Josephson junctions between two d-wave superconductors has been predicted to exhibit an anomalous periodicity. We have used the single-junction interferometer to investigate the CPR for these kinds of junctions in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ thin films. A remarkable amplitude of the $\ensuremath{\pi}$-periodical component of the CPR has been experimentally found, providing an additional source of evidence for the d-wave symmetry of the pairing state of the cuprates.

The real and imaginary part of the rf impedance of a phase-biased weak link based on YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films were investigated experimentally at 77 K. The investigated weak links were step-edge or bicrystal Josephson junctions incorporated into a washer type superconducting ring with a small inductance L=80...300 pH. The ring was inductively coupled to a tank circuit with a resonant frequency of about 23 MHz. The phase shift /spl alpha/ between the rf voltage and rf current through the tank circuit was found to be a convenient measure of the change of inductance of a weak link. From the dependence of /spl alpha/ on the magnetic flux in the ring one can deduce the current-phase relation I(/spl phi/)=I/sub c/ f(/spl phi/) of the weak link. The influence of thermal noise on f(/spl phi/) was estimated numerically using a Gaussian distribution of the noise amplitude. The experimentally observed dependence /spl alpha/(/spl phi/) for the bicrystal weak link can be sufficiently good described with f(/spl phi/)=sin (/spl phi/) and with a reasonable value of the noise amplitude. The step-edge junction shows remarkable deviation from the sinusoidal current-phase relation.

We demonstrate the narrow switching distribution of an underdamped Josephson junction from the zero to the finite voltage state at millikelvin temperatures. We argue that such junctions can be used as ultrasensitive detectors of the single photons in the GHz range, operating close to the quantum limit: a given initial (zero voltage) state can be driven by an incoming signal to the finite voltage state. The width of the switching distribution at a nominal temperature of about T = 10 mK was 4.5 nA, which corresponds to an effective noise temperature of the device below 60 mK.

A Lagrangian formalism is used to derive the Hamiltonian for a $\lambda$/4 resonator shunted by a current-biased Josephson junction. The eigenstates and the quantum dynamics of the system are analyzed numerically, and we show that the system can function as an efficient detector of weak incident microwave fields., Comment: 8 pages, 6 figures

Two basic physical models, a two-level system and a harmonic oscillator, are realized on the mesoscopic scale as coupled qubit and resonator. The realistic system includes moreover the electronics for controlling the distance between the qubit energy levels and their populations and to read out the resonator's state, as well as the unavoidable dissipative environment. Such rich system is interesting both for the study of fundamental quantum phenomena on the mesoscopic scale and as a promising system for future electronic devices. We present recent results for the driven superconducting qubit-resonator system, where the resonator can be realized as an LC circuit or a nanomechanical resonator. Most of the results can be described by the semiclassical theory, where a qubit is treated as a quantum two-level system coupled to the classical driving field and the classical resonator. Application of this theory allows to describe many phenomena for the single and two coupled superconducting qubits, among which are the following: the equilibrium-state and weak-driving spectroscopy, Sisyphus damping and amplification, Landau-Zener-St\"uckelberg interferometry, the multiphoton transitions of both direct and ladder- type character, and creation of the inverse population for lasing., Comment: 20 pages, 15 figures

We implement the impedance measurement technique in order to characterize superconducting structures designed for applications in quantum computing. We report an experimental study of the circuit consisting of three small Josephson junctions incorporated in a low-inductance superconducting loop. Measurements of the circuit response to an applied weak ac bias as a function of the external magnetic flux φe yield complete information on the properties of the circuit. We found that the system displays two metastable states. From experimental data, we have determined the magnetic field dependent Josephson energy U(φe) of these states, the potential barrier ΔU between these states, as well as the difference of the critical currents of the junction in the loop.

We present a detailed study of the microstructure of submicron Al/Al-O/Al Josephson junctions fabricated by the conventional shadow evaporation technique. The morphology of the dielectric Al-O layer, which plays the key role for junction transport properties, has been investigated by making use of high resolution electron microscopy. We demonstrate, that the flatness and thickness of the aluminum oxide layer strongly depends on its grain structure. The most pronounced thickness deviations are observed in the vicinity of so-called "triple points," where the grain boundary crosses the interlayer, forming a two-grain contact. Additionally we show that even for the single-grain contact, the Al/Al-O interface is not atomically flat, which can cause additional flicker noise at subkelvin temperatures. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3670003]

Applying a radio-frequency (rf) interference method, we were able to investigate the supercurrent across Josephson junctions down to some tens of nA, where the Josephson coupling energy is considerably smaller than the thermal energy at 4.2 K, the temperature of operation. We report about the measurement of small supercurrents by a rf interference method. The critical current Ic as well as the supercurrent-phase relation were derived from the measurement of the impedance of the phase-biased Josephson element at 10–30 MHz. Several single and double Nb/AlOx/Nb tunnel junctions with critical currents ranging from 50 nA to 3 μA have been studied. As expected, highly asymmetrical double junction structures exhibited a harmonic current-phase relation, while the latter indeed distinctly deviated from sin ϕ in the case of more balanced double junctions.

For a study of the supercurrent–phase relationship, we have fabricated Nb/Al/AlOx/Al/AlOx/Al/Nb-based Josephson junctions of low critical current density. At 4.2 K, junctions of a nominal area of 10×10 μm2 showed a nonhysteretic dc current–voltage characteristic with a critical current of about 10 μA. Similar junctions were part of washer-type rf superconductor quantum interference devices. These samples were inductively coupled to high-quality tank circuits for measurements at 4.2 K or at higher temperatures up to values close to the critical temperature of niobium. In this range, the current–phase relation, as deduced from impedance measurements of the phase-biased junction, did not deviate significantly from the sinusoidal shape.

The current-phase relation (CPR) of high-Tc ramp-edge Josephson junctions (YBa2Cu3O7−x/PrBa2Cu3O7/YBa2Cu3O7−x) was investigated experimentally in the temperature range 65

We use the density matrix formalism to analyze the interaction of interferometer-type superconducting qubits with a high quality tank circuit, which frequency is well below the gap frequency of a qubit. We start with the ground state characterization of the superconducting flux and charge qubits. Then, by making use of a dressed state approach we describe the qubits' spectroscopy when the qubit is irradiated by a microwave field which is tuned to the gap frequency. The last section of the paper is devoted to continuous monitoring of qubit states by using a DC SQUID in the inductive mode., Comment: 11 pages, 5 figures; the title and abstract are slightly changed; several typos are corrected; in order to make our argumentation more clear we added some comments in the introduction and other sections

The current-phase relation (CPR) of YBa2Cu3O7−x step-edge Josephson junctions was investigated experimentally at various temperatures. The investigated junctions were incorporated into a washer-shaped superconducting ring with inductance L∼300 pH. The ring was inductively coupled to a tank circuit with a resonance frequency between 9 MHz and 40 MHz. The CPR was obtained from the measurement of the impedance of the phase-biased junction. At high temperatures and low critical current the experimentally observed CPR is sinusoidal. The same results were obtained at low temperatures when the rms flux noise in the ring is negligible. In the case of larger critical current and finite noise, deviations of the apparent CPR from sinusoidal dependence were found. The deviations can be described by thermal fluctuations of the magnetic flux in the ring.

We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit--coupler interaction. The third junction gives the coupler a nontrivial current--flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from ~45 (antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior., Comment: REVTeX4, 4pp., 6 figs, 10 EPS figure files; N.B.: "Alec" is my first, and "Maassen van den Brink" my family name. v2: included discussion of a 2nd sample with sign-tunability v3:rewritten for publication

Experimental observation of the strong influence of an array of ferromagnetic nanodots on the critical current of a short overlap Josephson junction is reported. Pronounced commensurability effects are detected due to the presence of the additional peaks in the magnetic field induced diffraction pattern. The changes in the Fraunhofer pattern of the Josephson junctions are account for by the formation of Abrikosov vortices trapped in the electrodes which induce a phase inhomogeneity in the junction area., 5 pages, 4 figures

Superconductor∕normal conductor∕superconductor Josephson junctions with highly transparent interfaces are predicted to show significant deviations from sinusoidal supercurrent-phase relationships (CPR) at low temperatures. We investigate experimentally the CPR of a ballistic $\mathrm{Nb}∕\mathrm{In}\mathrm{As}(2\mathrm{DES})∕\mathrm{Nb}$ junction in the temperature range from $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}9\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ using a modified Rifkin-Deaver method. The CPR is obtained from the inductance of the phase-biased junction. Transport measurements complement the investigation. At low temperatures, substantial deviations of the CPR from conventional tunnel-junction behavior have been observed. The data are fitted with a model presented by Grajcar et al. [Physica C 372\char21{}376, 27 (2002)], yielding good agreement.

We have demonstrated strong antiferromagnetic coupling between two three-junction flux qubits based on a shared Josephson junction, and therefore not limited by the small inductances of the qubit loops. The coupling sign and magnitude were measured by coupling the system to a high-quality superconducting tank circuit. Design modifications allowing to continuously tune the coupling strength and/or make the coupling ferromagnetic are discussed., REVTeX 4, 4 pages, 5 figures; v2: completely rewritten, added finite-temperature results and proposals for ferromagnetic galvanic coupling

We present the first experimental results on a device with more than two superconducting qubits. The circuit consists of four three-junction flux qubits, with simultaneous ferro- and antiferromagnetic coupling implemented using shared Josephson junctions. Its response, which is dominated by the ground state, is characterized using low-frequency impedance measurement with a superconducting tank circuit coupled to the qubits. The results are found to be in excellent agreement with the quantum-mechanical predictions., Comment: REVTeX 4, 5pp., 7 EPS figure files. N.B.: "Alec" is my first, and "Maassen van den Brink" my family name. v2: final published version, with changed title, different sample micrograph, and several clarifications

Single layer autonomous high‐Tc dc superconducting quantum interference devices (SQUIDs) have been fabricated and tested. The SQUIDs were designed for application as a galvanometer. The current to be measured is injected directly in a microstrip segment of the SQUID loop. Step‐edge as well as bicrystal YBCO Josephson junctions were used. We consider two aspects: (i) optimization of the noise properties with respect to current resolution, and (ii) temperature dependence of the period of voltage‐flux relation. The SQUID inductance was calculated numerically taking into account the magnetic field penetration depth λ. The temperature dependence of λ(T) was obtained from experimental results and is found to be in good agreement with λ(T)≊λ(0)[1−(T/TC)2]−1/2.

We implemented experimentally an interferometer-type charge qubit consisting of a single-Cooper-pair transistor closed by a superconducting loop that is in flip-chip configuration inductively coupled to a radio frequency tank circuit. The tank permits to readout the qubit state, acting as inductance measuring apparatus. By applying continuous microwave power to the quantum device, we observed inductance alterations caused by redistributions of the energy level populations. From the measured data, we extracted the energy gap between ground and upper levels as a function of the transistor quasicharge as well as the Josephson phase across both junctions., Comment: 4 pages, 5 figures, revised version, to appear in PRB Rapid Comm

The quality of YBa2Cu3O7-delta blepitaxial grain boundary Josephson junctions has been improved and 'tunnel-like' characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favourable to isolate intrinsic d-wave induced effects. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for pi-circultry and qubits.

A detailed investigation of the magnetic response of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ grain-boundary Josephson junctions has been carried out using both radio-frequency measurements and scanning superconducting quantum interference device microscopy. In a nominally zero-field-cooled regime we observed a paramagnetic response at low external fields for $45\ifmmode^\circ\else\textdegree\fi{}$ asymmetric grain boundaries. We argue that the observed phenomenology results from the d-wave order-parameter symmetry and depends on Andreev bound states.

We propose to investigate flux qubits by the impedance measurement technique (IMT), currently used to determine the current--phase relation in Josephson junctions. We analyze in detail the case of a high-quality tank circuit coupled to a persistent-current qubit, to which IMT was successfully applied in the classical regime. It is shown that low-frequency IMT can give considerable information about the level anticrossing, in particular the value of the tunneling amplitude. An interesting difference exists between applying the ac bias directly to the tank and indirectly via the qubit. In the latter case, a convenient way to find the degeneracy point in situ is described. Our design only involves existing technology, and its noise tolerance is quantitatively estimated to be realistic., 6 pages, 11 figures, to appear in Phys.Rev.B

We study the phase dynamics and the resonance features of coupled Josephson junctions in layered superconductors and their manifestations in the current- voltage characteristics and temporal dependence of the electric charge in the superconducting layers. Results on the effect of the external radiation and shunting of the stack of Josephson junctions by LC-elements are presented. We discuss the ideas concerning the experimental observation of these resonances.