Single Josephson Junction Research Articles

Tunable Impedance Matching for Josephson Junction Reflection Amplifier

It has been recently demonstrated that a single Josephson junction with a properly engineered impedance environment can be used as a reflection amplifier approaching the quantum limit at microwave frequencies. In this article the authors describe an improved microwave setup for such a device. Voltage-tunable capacitors enable precise impedance matching across a wide range of operating points. Also, a branch-line coupler instead of a microwave circulator is used for the separation of the input and the output signals to make the system more compact and affordable. Furthermore, a custom-designed bias tee is introduced. The authors present characterization results of individual components as well as of the complete system. In particular, noise spectroscopy is used to study amplifier stability as a function of input matching.

A high-temperature superconducting monolithic microwave integrated Josephson down-converter with high conversion efficiency

A compact high-Tc superconducting monolithic microwave integrated circuit Josephson down-converter is presented. The circuit consists of a single Josephson junction mixer, a bandpass filter, a lowpass filter, and a resonator for local oscillator fabricated on a single 10 mm × 20 mm chip of YBa2Cu3O7−x film on MgO substrate. The down-converter demonstrates superior performance in terms of conversion efficiency, dynamic range, linearity, and low local oscillator power with stable operation from 20 to 77 K. A maximum conversion gain of −4.7 dB was measured at 20 K and −12.8 dB at 70 K.

Development and metrological applications of Josephson arrays at PTB

The discovery of the Josephson effects 50 years ago initiated a revolution for electrical voltage metrology. This revolution started with single Josephson junctions delivering a few millivolt at most. Meanwhile, highly integrated series arrays containing more than 10 000 or even 300 000 junctions have been developed and fabricated for output voltages up to 10 V. Josephson voltage standards are nowadays used in many laboratories worldwide for dc applications. After their adoption for the representation of the unit of voltage in 1990, the focus of research shifted towards programmable Josephson series arrays. The increasing interest in highly precise ac voltages resulted in new developments for their metrological applications in the last 15 years. This paper summarizes the principal contributions from PTB to the present state of Josephson voltage standards with particular focus on developments and applications for ac standards in metrology and our proof-of-concept demonstrations. The presentation includes the two most promising versions of ac standards, being the programmable Josephson voltage standard based on binary divided series arrays and the Josephson arbitrary waveform synthesizer based on a pulse-driven series array.

Superconducting qubit in a waveguide cavity with a coherence time approaching 0.1 ms

We report a superconducting artificial atom with an observed quantum coherence time of T2*=95us and energy relaxation time T1=70us. The system consists of a single Josephson junction transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.

Nano Superconducting QUantum Interference Device Sensors for Magnetic Nanoparticle Detection

In this paper, Superconducting QUantum Interference Devices (SQUIDs) based on single layer Nb nanobridge Josephson junctions are described. Devices, with loop area ranging from 4 to 0.5 microm2, have been patterned by Electron Beam Lithography (EBL) in a 20 nm thick Nb layer, achieving a responsivity of about 30 microA/phi0. Magnetization measurements have been performed via switching current measurements at a temperature T = 4.2 K. Preliminary detection of Silica-magnetite (Fe3O4-SiO2) core/shell nanoparticle cluster has been proven.

Resonant Tunneling in a Polymer with Charge Injection Doping

It is shown that a supercurrent in disordered polymers placed between superconductor electrodes may be explained by resonant tunneling of Coopers pairs along polymer channels formed due to electrification effect. Polymer film thickness where supercurrent one may to observe depends on superconducting coherence length in superconductor. The more coherence length in superconductor electrode the more is a polymer film thickness where superconductor exists. Shapiro steps induced by microwave (f = 9.3 GHz) radiation correspond to a single Josephson junction.

Identification of parameters with different orders of magnitude in chaotic systems

The synchronization and parameter identification of six unknown parameters in a chaotic neuron model, which one parameter (about 0.006) is 3 orders of magnitude smaller than the others (about 1–5), is investigated by using Lyapunov stability theory and adaptive synchronization in detail. Two gain coefficients (δ1, δ2) are introduced into the Lyapunov function to obtain certain optimized controllers and parameter observers. A selectable amplification factor k 0 is presented using scale conversion and it is used to improve the accuracy of parameter estimation with the smallest order. The parameter space for gain coefficient (δ) versus amplification factor k 0, and the parameter space δ1 versus δ2 at certain fixed amplification factor k 0 are calculated numerically. It is found that the selection values of optimized gain coefficients and amplification factor are critical to estimate the six unknown parameters, particularly for the smallest unknown parameters with an order 0.001. The extensive numerical results show that it is more effective to estimate the smallest unknown parameter r when the two gain coefficients δ1 and δ2 are given the same value and a higher amplification factor k 0 is used. It could be useful to estimate the unknown parameters with large deviation of order magnitude, such as a single chaotic Josephson junction coupled to a Resonant tank and other chaotic systems with potential application [Z.Y. Wang, H.Y. Liao, and S.P. Zhou, Study of the DC biased Josephson junction coupled to a Resonant tank, Acta. Phys. Sin. 50(10) (2001), pp. 1996–2000 (in Chinese)].

Fabrication and characterization of single Nb/NbxSi1-x/Nb Josephson junction for voltage standard

The core device of Modern programmable Josephson voltage standard is Josephson junction array. The most advantageous Josephson junction array is Nb/NbxSi1-x/Nb material array. The advantages of Nb/NbxSi1-x/Nb material Josephson junction are that three-layer film production process is simple, Nb and NbxSi1-x etching processes are the same and NbxSi1-x potential barrier layer components can be easily adjusted. In this paper, we investigate the NbxSi1-x/Nb single Josephson junction in National Institute of Metrology. Through measuring the dc current-voltage characteristics under low temperature (4.2 K), superconducting tunneling current and a zero voltage state jumping to voltage state are observed clearly, finally the measurement results are discussed. The work is the first study on Nb/NbxSi1-x/Nb single Josephson junction in China.

Spin interference of holes in silicon nanosandwiches

Spin-dependent transport of holes is studied in silicon nanosandwiches on an n-Si (100) surface which are represented by ultranarrow p-Si quantum wells confined by δ-barriers heavily doped with boron. The measurement data of the longitudinal and Hall voltages as functions of the top gate voltage without an external magnetic field show the presence of edge conduction channels in the silicon nanosandwiches. An increase in the stabilized source-drain current within the range 0.25–5 nA subsequently exhibits the longitudinal conductance value 4e2/h, caused by the contribution of the multiple Andreev reflection, the value 0.7(2e2/h) corresponding to the known quantum conductance staircase feature, and displays Aharonov-Casher oscillations, which are indicative of the spin polarization of holes in the edge channels. In addition, at a low stabilized source-drain current, due to spin polarization, a nonzero Hall voltage is detected which is dependent on the top gate voltage; i. e., the quantum spin Hall effect is observed. The measured longitudinal I–V characteristics demonstrate Fiske steps and a negative differential resistance caused by the generation of electromagnetic radiation as a result of the Josephson effect. The results obtained are explained within a model of topological edge states which are a system of superconducting channels containing quantum point contacts transformable to single Josephson junctions at an increasing stabilized source-drain current.

Double rf-SQUIDs Operating in a Non-Adiabatic Regime: A Dream Comes True?

Abstract At a time when the supremacy of SQUIDs as the most sensitive magnetometers is being challenged by the recent progress of optically pumped atomic magnetometers operating at room temperature, looking into new opportunities to further improve SQUIDs sensitivity would probably be most welcomed by the SQUID community. By far the most promising such opportunity is to operate a double (D) rf SQUID (an rf-SQUID in which a dc-SQUID is used instead of a single Josephson junction) in a non-adiabatic regime at frequencies of about a few GHz. At these relatively high frequencies required to assure an ultra-high SQUID sensitivity the SQUID-design complexity and the cost of the electronics involved (current generator and SQUID amplifier) are still kept at reasonable levels. The advantages of using this concept, as presented in my contribution in more detail now, are overwhelming. Ironically, although this idea in its preliminary shape has been around for some time now [1], the opportunity has never been explored experimentally! I am just wondering what else needs to be done to get this message across the SQUID community and have this concept finally tested experimentally?

Phase-Diffusion in Switching Process of Underdamped Josephson Junctions

We measured the switching from a superconducting state to a finite voltage state in a dc SQUID, which is equivalent to a single Josephson junction with tunable critical current Ic. The width and the mean of the switching current distribution depend on the effective temperature Teff and Ic. The phase-diffusion was observed by investigating the switching current distribution as a function of Teff. It is found that the crossover from the thermal activation to the phase-diffusion scaling with the ratio of Ic and Teff, clarifying the physical picture of the phase-diffusion. We developed an analytical multi-retrapping model which can address the experimental results very well.

Skewness in the current-phase relation of double-barrier Josephson junctions

The current-phase relations of a double-barrier Josephson junction with a thin intermediate electrode show deviations from the usual sinusoidal dependence of a single Josephson junction. The skewness of these curves can be analytically found for small values of the coupling between the two outer electrodes of the double-barrier Josephson junction.

Charge transport through ultrasmall single and double Josephson junctions coupled to resonant modes of the electromagnetic environment

We have investigated charge transport in ultrasmall superconducting single and double Josephson junctions coupled to resonant modes of the electromagnetic environment. We observe pronounced current peaks in the transport characteristics of both types of devices and attribute them to the process involving simultaneous tunneling of Cooper pairs and photon emission into the resonant modes. The experimental data is well reproduced with the theoretical models.

Transport dynamics with alternate Cooper-pair and quasiparticle tunnelings in one-dimensional charge Josephson arrays

For a single weak-coupled Josephson junction, stochastic Cooper-pair tunnelings drive the superconducting current on low voltage biases, while quasiparticle tunnelings stimulate the normal current on high biases above the superconducting gap voltage. Considering these interactive dynamics simultaneously in one-dimensional weak-coupled Josephson arrays in electrodynamic environments, the theoretical work by rate equations is structured herein, and the charge transport associated with short-range (Cooper pair and quasiparticle) and long-range (Cooper-pair soliton and quasiparticle soliton) behaviors is analyzed.

Readout for phase qubits without Josephson junctions

We present a readout scheme for phase qubits which eliminates the read-out superconducting quantum interference device so that the entire qubit and measurement circuitry only require a single Josephson junction. Our scheme capacitively couples the phase qubit directly to a transmission line and detects its state after the measurement pulse by determining a frequency shift observable in the forward scattering parameter of the readout microwaves. This readout is extendable to multiple phase qubits coupled to a common readout line and can in principle be used for other flux biased qubits having two quasistable readout configurations.

Phase-charge duality in Josephson junction circuits: Role of inertia and effect of microwave irradiation

We investigate the physics of coherent quantum phase slips in two distinct circuits containing small Josephson junctions: (i) a single junction embedded in an inductive environment and (ii) a long chain of junctions. Starting from the standard Josephson Hamiltonian, the single junction circuit can be analyzed using quasi-classical methods; we formulate the conditions under which the resulting quasi-charge dynamics is exactly dual to the usual phase dynamics associated with Josephson tunneling. For the chain we use the fact that its collective behavior can be characterized by one variable: the number $m$ of quantum phase slips present on it. We conclude that the dynamics of the conjugate quasi-charge is again exactly dual to the standard phase dynamics of a single Josephson junction. In both cases we elucidate the role of the inductance, essential to obtain exact duality. These conclusions have profound consequences for the behavior of single junctions and chains under microwave irradiation. Since both systems are governed by a model exactly dual to the standard resistively and capacitively shunted junction model, we expect the appearance of current-Shapiro steps. We numerically calculate the corresponding current-voltage characteristics in a wide range of parameters. Our results are of interest in view of a metrological current standard.

Phase qubit on a superconducting single-junction interferometer

The energy spectrum of a phase quantum bit (qubit) implemented on a superconducting interferometer with a single Josephson junction has been studied in terms of the Hamilton formalism. An analytical formula for ground level splitting in the qubit spectrum is obtained and analyzed.

Thermal effects in microwave current-induced weak link switching in YBCO thinfilms

In this study, we have measured the microwave power dependence of the surface resistance (Rs) of aYBa2Cu3Ox (YBCO) thin filmdeposited on an LaAlO3 substrate up to superconductivityquenching microwave power levels. The time-domaintechnique was used to monitor the electromagnetic response dynamics of a 14 GHzTE011 dielectric sapphire resonator to pulsed microwave signals. Local and global heating inducednonlinearity was observed. This nonlinearity was analyzed using a simple analytical modelto describe the charging process of the dielectric resonator in a YBCO thin film thatincluded a single Josephson junction weak link. From the analysis of the switching-onweak links, we confirm the presence of heating on weak links even in the linearcase and we also estimate that the local heating rate is of the order of a fewmicroseconds.

Comparative study of macroscopic quantum tunneling inBi2Sr2CaCu2Oyintrinsic Josephson junctions with different device structures

We investigated macroscopic quantum tunneling (MQT) of ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{y}$ intrinsic Josephson junctions (IJJs) for two device structures. One is a small mesa, which is a few nanometers thick with only two or three IJJs, and the other is a stack of a few hundred IJJs in a narrow bridge structure. The experimental results regarding the switching-current distribution for the first switch from the zero-voltage state were in good agreement with the conventional theory for a single Josephson junction, indicating that the crossover temperature from thermal activation to the MQT regime for the former device structure was similar to that for the latter device structure. Together with the observation of multiphoton transitions between quantized energy levels in the MQT regime, these results strongly suggest that the observed MQT behavior is intrinsic to a single IJJ in high-${T}_{c}$ cuprates and is independent of the device structure. The switching-current distribution for the second switch from the first resistive state, which was carefully distinguished from the first switch, was also compared with respect to the two device structures. In spite of the differences between the heat transfer environments, the second switch exhibited a similar temperature-independent behavior for both devices up to a much higher temperature than the crossover temperature for the first switch. We argue that this cannot be explained in terms of self-heating caused by dissipative currents after the first switch. As possible candidates for this phenomenon, the MQT process for the second switch and the effective increase in the electronic temperature due to the quasiparticle injection are discussed.

Investigation of resonant and transient phenomena in Josephson junction flux qubits

We present an analytical and computational study of resonances and transient responses in a classical Josephson junction system. A theoretical basis for resonances in a superconducting loop with three junctions is presented, outlining both the direct relationship between the dynamics of single- and multijunction systems and the direct relationships between observations of the classical counterparts to Rabi oscillations, Ramsey fringes, and spin-echo oscillations in this class of systems. We show simulation data along with analytical analyses of the classical model, and the results are related to previously reported experiments conducted on three junction loops. We further investigate the effect of off-resonant microwave perturbations to, e.g., the Rabi-type response of the Josephson system, and we relate this response back to the nonlinear and multivalued resonance behavior previously reported for a single Josephson junction. The close relationships between single and multijunction behaviors demonstrate the underlying dynamical mechanism for a whole class of classical counterparts to expected quantum-mechanical observations in a variety of systems, namely, the resonant and transient behavior of a particle in an anharmonic potential well with subsequent escape.