Hysteretic Josephson Junctions Research Articles

Stability of thermally bistable states and their switching in superconducting weak link

Superconducting weak link (WL), acting as a Josephson junction (JJ), is one of the widely used elements in superconductor science and quantum circuits. A hysteretic JJ with robust switching between its superconducting and resistive state is an excellent candidate for single-photon detection. However, the ubiquitous fluctuations in the junction strongly influence the stability of the states and, thus, the transition from one to the other. Here, we present an experimental study of switching statistics of critical and retrapping currents of a JJ based on niobium WL in its hysteretic regime. The mean lifetimes of the two metastable states, namely, the zero-voltage superconducting state and finite-voltage resistive state, are estimated from the distributions. Further, close to the hysteresis crossover temperature, observed telegraphic noise in the time domain due to random switching between the states provides their lifetimes directly. We present a thermal model introducing a double-well (bistable) feature with an intriguing quantity with respect to the devices’ temperature states. The effects of temperature fluctuations on the stability of the states are shown. We discuss our results toward further improvement of the efficiency of superconducting WL or nanowire single-photon detectors.

Non-Resonant Microwave Absorption in SmFeAsO 0.80 F 0.20: Line Shape and Structure Evolution with Temperature

‘Non-resonant Microwave Absorption’ (NRMA) or the ‘Low field microwave absorption’ (LFMA) measurements on high-quality polycrystalline SmFeAsO0.80F0.20 superconducting sample were carried as functions of temperature and microwave power. The LFMA line shape is complex with two peaks namely; broad peak 1 and narrow peak 2 akin to one reported in SmFeAsO0.88F0.12 as reported by Onyancha et al (Supercond. Nov. Magn. 28, 2927–2934, 2015). This unquestionably illustrates that these peaks are a common feature in F-doped SmFeAsO. The LFMA signal as a function of temperature reveals that T c − T ∗ = 1K in SmFeAsO0.80F0.20 compared to 4 K in SmFeAsO0.88F0.12 (T ∗ is the characteristic temperature at which the narrow peak appears as we cool down the sample below T c); hence inferring that the narrow peak is fluorine doping dependent. Furthermore, LFMA signal evolution with microwave power does not show phase reversal (anomalous absorption) at 2.227 mW which is a stark contrast to what was observed in SmFeAsO0.88F0.12 as reported by Onyancha et al (Physica C: Supercond. Appl. 533:49–52, 2017). The absence of phase reversal within measured microwave power indicates presence of hysteretic Josephson junction. These findings establish few non-superconducting inclusions in SmFeAsO0.80F0.20 system.

Quantum Nondemolition Measurement of a Superconducting Qubit in the Weakly Projective Regime

Quantum state detectors based on switching of hysteretic Josephson junctions biased close to their critical current are simple to use but have strong backaction. We show that the backaction of a dc-switching detector can be considerably reduced by limiting the switching voltage and using a fast cryogenic amplifier, such that a single readout can be completed within 25ns at a repetition rate of 1MHz without loss of contrast. Based on a sequence of two successive readouts we show that the measurement has a clear quantum nondemolition character, with a QND fidelity of 75%.

Detecting non-Gaussian current fluctuations using a Josephson threshold detector

We employ a hysteretic Josephson junction in on-chip detection of shot noise generated in a tunnel junction. The switching rate of the detector from its supercurrent state displays an asymmetry with respect to the direction of the mean current through the noise source. The detector operates in a high-frequency regime with the relevant bandwidth set by its plasma frequency ω p / 2 π ≃ 50 GHz . Assuming close to Gaussian noise, we discuss a model relating the measured asymmetry to the third cumulant of shot noise.

Wideband Detection of the Third Moment of Shot Noise by a Hysteretic Josephson Junction

We use a hysteretic Josephson junction as an on-chip detector of the third moment of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the present experiment. The third moment of shot noise results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We analyze the observed asymmetry assuming adiabatic response of the detector.

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.

Shot-Noise-Driven Escape in Hysteretic Josephson Junctions

We have measured the influence of shot noise on hysteretic Josephson junctions initially in the macroscopic quantum tunneling regime. The escape threshold current into the resistive state decreases monotonically with increasing average current through the scattering conductor, which is another tunnel junction. Escape is predominantly determined by excitation due to the wideband shot noise. This process is equivalent to thermal activation (TA) over the barrier at effective temperatures up to about 4 times the critical temperature of the superconductor. The presented TA model is in excellent agreement with the experimental results.

Josephson Junction as a Detector of Poissonian Charge Injection

We propose a scheme of measuring the non-Gaussian character of noise by a hysteretic Josephson junction in the macroscopic quantum tunneling regime. We model the detector as an (under)damped LC resonator. It transforms Poissonian charge injection into current through the detector, which samples the injection statistics over a floating time window of length approximately Q/omega(J), where Q is the quality factor of the resonator and omega(J) its resonance frequency. This scheme ought to reveal the Poisson character of charge injection in a detector with realistic parameters.

Characterization of an LC-isolated Josephson junction qubit

The energy states of a well-isolated hysteretic Josephson junction in the "phase" regime can be used as a qubit. The state of the junction can be determined by measuring when the junction switches from the zero-voltage (qubit) state to the running voltage state, since different levels have different switching rates. The experimental challenge is to provide sufficient electrical isolation of the junction from wires that must be attached to provide bias current and to measure switching events. This isolation must be effective at frequencies around the energy level spacing, in our case from 3-8 GHz. We report on the design and measurement of isolated moderate-to-high-Q junctions using a resonant isolation scheme of a series inductance and capacitive shunt. Microwave activation measurements of the Al-AlO/sub x/-Al junctions at 70 mK were used to gauge the isolation effectiveness and to show the effect of current noise from the isolation resonance on the junction coherence.

MAGNETIC FIELD CONTROLLED SUPERCONDUCTING - RESISTIVE AND RESISTIVE - SUPERCONDUCTING TRANSITIONS IN HIGHLY HYSTERETIC JOSEPHSON JUNCTIONS

We report measurements of the switching induced by a rapidly varying magnetic field in a single, small, fully hysteretic Josephson junction. We demonstrate that the variation of the magnetic field, generated by a current step in a control line deposited on the top of the junction, can control the transitions from Josephson branch to the resistive branch and vice versa. The transition from resistive branch to Josephson branch is not trivial for an hysteretic junction. With a suitable choice of the magnetic pulse shape, a "deterministic flip-flop regime" can be achieved in this system.

Return current statistics in extended Josephson junctions

In the last few years the return current, i.e., the value of the bias current where an hysteretic Josephson junction switches back to zero voltage state, has obtained interest in view of its connections with the evaluation of losses for macroscopic quantum coherence experiments and the following development of q-bit devices. Here we study carefully the problem of the return current by means of numerical simulations of associated Langevin equations in both very small and intermediate length Josephson junctions. Comparison with theoretical lifetime, fluctuations, and recent experiments are carefully carried out. The results show that the effects of finite length are important in the correct description of the return switch.

Direct measurement of the Josephson supercurrent in an ultrasmall Josephson junction.

We have measured the supercurrent flowing through a nonhysteretic, ultrasmall, voltage-biased Josephson junction. In contrast with experiments performed so far on hysteretic Josephson junctions, we find a supercurrent peak whose maximum I(s max) increases as the temperature T decreases. The asymptotic T = 0 value of I(s max) agrees with the junction Ambegaokar-Baratoff critical current, as predicted by theory.

Return current in hysteretic Josephson junctions: Experimental distribution in the thermal activation regime

We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly different from what is expected. The disagreement is found even in zero-applied magnetic field and it cannot be attributed to external noise in the system. From the experimental statistical properties, we find values for the effective dissipation much lower than those obtained from the deterministic curves, a result which could be of interest in experiments on the observation of macroscopic quantum phenomena.

Gain-frequency characteristics of transistors based on flux flow in hysteretic long Josephson junctions (LJJ)

Transistors based on one dimensional flux flow in hysteretic long Josephson junctions (LJJ) and deriving output from the spatial average over the length can be modeled as a section of transmission line. Time domain analyses made on these transmission line sections with both the ends terminated with typical resistance values reveal a fundamental gain-frequency relationship unique to all devices whose output voltage is equal to the spatial average over the length of the junction. The maximum frequency of a transistor depends on the transit time of carriers and the parasitic elements. The analysis shows that the frequency response of a flux flow transistor is related but not equal to the inverse of the transit time of fluxons. Secondly the step response of these transmission line sections varies linearly with time, which indicates a slew rate limitation. The slew rate that affects the high frequency response of a flux flow device is shown to be a function of the critical current density of the junction and material parameters. Analysis made on niobium-lead junctions indicates that the slew rate peaks at an optimum current density level. The results of these analyses and performance comparisons are presented.

Development and test of Josephson devices for an experiment of macroscopic quantum coherence

We report on the development and test of Josephson devices, namely tunnel junctions and r.f.-SQUIDs (Superconducting QUantum Interference Devices), in the framework of an experiment to detect macroscopic quantum coherence (MQC, started in 1994). To meet the stringent requirements imposed by the experiment, one needs high-quality, hysteretic Josephson junctions, with an almost ideal behavior for what concerns dissipation, even at very low temperature (down to the mK range). Such junctions must then be used inside a superconducting loop, to realize the r.f.-SQUID which is the source of the macroscopic state to be tested. We fabricated both types of devices and tested their characteristics, included the response to thermal activation, at 4.2 K and, for the junctions, down to 300 mK.

Fabrication of capacitively shunted high Tc Josephson junctions

Hysteretic high Tc Josephson junctions were fabricated by shunting grain boundary YBa2Cu3O7 junctions with a monolithic Au film capacitor. The junctions were made by pulsed laser deposition with ion milling, and the shunt capacitor by rf sputter deposition and lift-off. The McCumber parameter βc obtained from the measured current-voltage curves was much smaller (<6) than the calculated values (3×103−3×104). The discrepancy is believed to be due to that the series resistance of the Au film dominates the capacitive reactance in the high voltages and thus reduces the hys resistance.

The role of quantized energy levels in the macroscopic quantum behavior of Josephson junctions

The effects of the presence of quantized levels in the problem of the (V=0)-state decay in Josephson junctions have been investigated. In considering the dynamics of the junction in the quantum picture, a preliminary study of the classical thermal fluctuation is necessary especially for the determination of the relevant junction parameters to be used in the quantum limit. Measurements of the lifetime of the zero-voltage state in a single hysteretic Josephson junction show clear evidence of two different damping regimes. Results are obtained for a wide range of damping within the underdamped region because the damping resistance depends exponentially on inverse temperature as does the junction quasiparticle resistance. This latter aspect is relevant because or allows to obtain a damping level that decreases with temperature, and hence, at low temperature we are able to study an extremely underdamped system. Many new interesting aspects related to the quantum behavior of the junction can be observed with such a low damping level. It is shown that a fast sweep of a Josephson tunnel junction can reveal the presence of quantum energy levels in the associated washboard potential. Peculiar effects, which can be observed on P(I) and Г(I), are obtained by numerical integration of the entire time dependent master equation describing the dynamics of the statistical population of each quantum level. The curves obtained predict the results of possible new experiments. Finally, we discuss, within the well-established quantum picture of the junction, the idea of a resonant macroscopic quantum tunneling between levels with the same energy in neighboring wells of the potential shape describing the junction. This process produces small voltage spikes in the current-voltage characteristics, as well as peculiar peaks in the current switching distributions at given current values, which can be measured in junctions and SQUIDs with suitable parameters.

RF coupling to single crystal BSCCO under c-axis bias

We have explored rf coupling to BSCCO single crystals with c-axis bias currents over a wide range of frequencies. A structure was developed that afforded the necessary impedance transformations at several frequencies from dc to 35 GHz. At low frequencies, connections were made to a coaxial cable. At X-band, the sample was placed in front of a short section of waveguide, forming a resonator. Above 12 GHz, the sample mount formed a coplanar resonator. At 35 GHz it was used as the end plate of a cylindrical cavity with degenerate mode spoilers and a loaded Q-factor of 3800 at 38.3 GHz. Fabricated BSCCO samples showed evidence of hysteretic Josephson junction stack behavior at 77 K. Application of low frequency rf signals caused steps to appear in the sample I-V. At frequencies above 6.4 GHz only a reduction of critical current was observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Intrinsic Josephson effects in layered superconductors

The large anisotropy and the extremely short coherence lengths of the high-T c superconductors suggest, that the layered crystal structure is mapped onto a periodic order parameter modulation. Even an ideal single crystal should be a stack of Josephson junctions. The measurements were performed on small single crystals of (Bi 1-xPb x) 2Sr 2CaCu 2O 8, Tl 2Ba 2Ca 2Cu 3O 10, and Pr 2−xCe xCuO 4. For current flow perpendicular to the CuO 2 planes, the current-voltage characteristics show a series connection of hysteretic Josephson junctions. For Bi 2Sr 2CaCu 2O 8, the modulation of the critical current in a magnetic field is consistent with a junction spacing of 1.5 nm, i.e. the distance between two CuO 2 bilayers. By the incoherent microwave emission of the AC Josephson currents between 4.5 and 95 GHz, the number of emitting junctions can be counted. Indeed, the single crystals consist of a stack of Josephson junctions. Every pair of CuO 2 bilayers forms a Josephson junction. Similar results have been obtained very recently on single crystals of the organic superconductor κ-(BEDT-TTF) 2Cu(NCS) 2. This observation supports the conclusion that in any layered superconductor with sufficiently high anisotropy the superconducting order parameter is spatially inhomogeneous a priori.

Frequency-to-voltage converter based on Bloch oscillations in a capacitively coupled GaAs-GaxAl1-xAs quantum well.

A charged capacitor which is placed with a semiconductor in a closed loop will normally discharge in a characteristic time [tau]=1/[ital RC]. We show, however, that if a semiconductor of high purity is subjected to a rf bias with frequency [omega], discharge does not occur if the voltage across the capacitor is proportional to an integer multiple of [h bar][omega]/[ital e]. Thus a rf frequency may be converted to a dc voltage. The ac to dc conversion is made possible by the interplay between Bloch oscillations induced by the electric field of the external rf source and those induced by the field from the charge residing on the capacitor. The process resembles the frequency-to-voltage conversion by a hysteretic Josephson junction, except that the evolution of the Josephson phase is replaced by the evolution of the crystal momentum of conduction electrons in the semiconductor. We discuss the parameters and conditions for which this behavior may be observed.