Macroscopic Quantum Tunneling Research Articles

Collective macroscopic tunneling in intrinsic Josephson junctions

A theory for photon-assisted macroscopic quantum tunneling (MQT) in a stack of capacitively-coupled intrinsic Josephson junctions is proposed. The theory predicts how the quantized energy-levels in the N-junction stacked system distribute. Furthermore, the theory reveals that the MQT escape rate from the single-plasmon states to the uniform voltage ones under microwave irradiation is proportional to the square of the junction number (N2). This result is consistent with the recent observation in Bi-2212 intrinsic Josephson junctions.

Macroscopic Quantum Tunneling and Resonant Activation of Current Biased Intrinsic Josephson Junctions in Bi-2212

The utilization of a high-Tc superconductor for implementing a superconducting qubit is to be expected. Recent researches on the quantum property of Josephson junctions in high-Tc superconductors indicate that the low energy quasiparticle excitation is weak enough to observe the macroscopic quantum tunneling. Therefore, a detailed study on the quantum property of high-Tc Josephson junctions becomes more important for applications. We show our experimental results of the macroscopic tunneling of current biased intrinsic Josephson junctions in Bi-2212 and its resonant activation in the presence of microwave radiation.

Measurement System for Switching Current Distribution in Intrinsic Josephson Junctions

A measurement system is developed to observe the switching current distribution in Bi 2 Sr 2 CaCu 2 O 8+δ intrinsic Josephson junctions (IJJ's). We have designed the frequency responses of filters and cables to achieve the compatibility of sufficient isolation at high frequency region and accurate detection of the distribution at low frequency region. The temperature dependence of the switching current distributions measured on a IJJ by the present system agrees well with the theoretical calculation in the temperature range from 70 mK to 5 K. The consistency of the crossover temperature between experimental result and calculation suggests that the designed measurement system succeeded in observing the macroscopic quantum tunneling process.

Experimental Designs for Binary Data in Switching Measurements on Superconducting Josephson Junctions

Summary We study the optimal design of switching measurements of small Josephsonjunction circuits which operate in the macroscopic quantum tunnelling regime. In the experiment, sequences of current pulses are applied to the Josephson junction sample, while the voltage over the structure is monitored. The appearance of a voltage pulse to a single applied current pulse, being governed by the laws of quantum mechanics, is purely random. Starting from the D-optimality criterion we derive the optimal design for the estimation of the unknown parameters of the underlying Gumbel-type distribution. As a practical method for the measurements, we propose a sequential design that combines heuristic search for initial estimates and maximum likelihood estimation. The design presented has immediate applications in the area of superconducting electronics, implying faster data acquisition. The experimental results presented confirm the usefulness of the method.

Quantum Terahertz Electrodynamics and Macroscopic Quantum Tunneling in Layered Superconductors

We derive a quantum field theory of Josephson plasma waves (JPWs) in layered superconductors, which describes two types of interacting JPW bosonic quanta (one heavy and one lighter). We propose a mechanism of enhancement of macroscopic quantum tunneling (MQT) in stacks of intrinsic Josephson junctions. Because of the long-range interaction between junctions in layered superconductors, the calculated MQT escape rate Gamma has a nonlinear dependence on the number of junctions in the stack. We show that the crossover temperature between quantum and thermal escape increases when increasing the number of junctions. This allows us to quantitatively describe striking recent experiments in Bi2Sr2CaCu2O8+delta stacks.

Study of macroscopic quantum tunnelling inBi2Sr2CaCu2O8+δ intrinsic Josephson junctions

We report an experimental observation of the macroscopic quantum tunnelling (MQT) in ad-wave high-temperature superconductor (HTSC)Bi2Sr2CaCu2O8+δ intrinsic Josephson junction (IJJs). It is well known that the current-biased Josephsontunnel junction provides an ideal stage for studying a variety of macroscopicquantum phenomena such as MQT. They have been studied and observed inlow-temperature superconductor (LTSC) Josephson junctions since the early 1980s andhave been established well by now. On the other hand, in the case of HTSC thed-wave pairing symmetry allows for dissipative quasiparticle excitations within thesuperconducting energy gap, which is expected to severely interfere with an observation ofMQT. Contrary to this naive conception, our experimental results have explained that theeffects of the nodal quasiparticles are not strong enough to obscure the observation of theMQT. Furthermore, we found that the MQT in IJJs was observed at approximately 1 K,which was ten times higher than that of LTSC Josephson junctions. This higherclassical-to-quantum crossover temperature is due to their high plasma frequency. We alsodiscuss the resonant activation of IJJs in the presence of microwave radiation.

Macroscopic quantum tunneling in globally coupled series arrays of Josephson junctions

We present a quantitative analysis of an escape rate for switching from the superconducting state to a resistive one in series arrays of globally coupled Josephson junctions. A global coupling is provided by an external shunting impedance. Such an impedance can strongly suppress both the crossover temperature from the thermal fluctuation to quantum regimes, and the macroscopic quantum tunneling (MQT) in short Josephson junction series arrays. However, in large series arrays we obtain an enhancement of the crossover temperature, and a giant increase of the MQT escape rate. The effect is explained by excitation of a spatial-temporal charge instanton distributed over a whole structure. The model gives a possible explanation of recently published experimental results on an enhancement of the MQT in single crystals of high-${T}_{c}$ superconductors.

Macroscopic Quantum Phenomena in High Critical Temperature Superconducting Josephson Junctions

YBa2Cu3O7−δ grain boundary bi-epitaxial Josepshon junctions (JJs) allow a very clear demonstration of Josephson current variation with the misorientation angle, consistent with the d-wave symmetry of the superconducting order parameter in cuprate, high temperature superconductors. Our bi-epitaxial junctions show a strong suppression of the first harmonic, I1 sin o, of the current phase relation when tunneling from a lobe into a node of the superconducting gap function. In these configurations, the contribution of the second harmonic, I2 sin 2o, becomes of the same magnitude as the first one, giving rise to a characteristic two-well Josephson potential as a function of phase o instead of the usual single well. This characteristic intrinsic property has suggested proposals of a new class of qu-bit named “quiet” because of the existence a spontaneously degenerate fundamental state without the need of applying an external field. Our experiments probe the macroscopic quantum properties in a d-wave Josephson junction by measuring macroscopic quantum tunneling and energy level quantization. The switching current out of the zero voltage state is measured as a function of temperature down to 20 mK. The temperature variation of the width of an ensemble of switching events goes over from one, which is characteristic of a thermal activation of phase fluctuations to a temperature independent width which is a token of quantum tunneling of the phase. The transition regime is affected by the two-well potential in a 45∘ misorientation junction as the second harmonic term gives rise to additional thermal transitions. The difference between quantized energy levels in the harmonic potential was determined by microwave spectroscopy. From the broadening of energy levels, it was possible to extract a Q-value of about 40 for the phase oscillations. The relatively high Q indicates quantum coherence over a sizeable time in d-wave junctions and gives hopes for a realization of a “quiet” high-Tc qu-bit. The contributions of V. L. Ginzburg to several different fields of physics are impressive and long standing. In superconductivity the Ginzburg–Landau theory, for instance, still represents a very powerful approach to model a huge number of different physical systems. High Temperature Superconductors (HTS) have strongly influenced research of the last 20 years and their d-wave order parameter symmetry represents one of the most intriguing features from both the fundamental point of view and some types of innovative long-term applications.

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.

Theory of macroscopic quantum tunnelling and dissipation inhigh-Tc Josephson junctions

We have investigated macroscopic quantum tunnelling (MQT) in in-planehigh-Tc superconductor Josephson junctions and the influence of the nodal-quasiparticle and zeroenergy bound states (ZES) on MQT. We have shown that the presence of ZES at theinterface between the insulator and the superconductor leads to strong Ohmic quasiparticledissipation. Therefore, the MQT rate is noticeably suppressed in comparison withc-axis junctions in which ZES are completely absent.

Collective dynamics of macroscopic quantum tunnelling in layeredhigh-Tc superconductors

In order to construct a theory for photon-assisted macroscopic quantum tunnelling (MQT)in intrinsic Josephson junctions, we revisit the model of capacitively coupledintrinsic Josephson junctions and clarify how the quantized energy levels in theN-junction stacked system distribute inside a valley of the Josephson sinusoidal potential.Furthermore, we reveal that the MQT escape rate from the single-plasmon states to the uniformvoltage ones by microwave irradiation is proportional to the square of the junction number(N2) in multi-photon processes. This result is consistent with the recent finding inBi-2212 intrinsic Josephson junctions by Jin et al 2006 Phys. Rev. Lett. 96 177003.

Experimental study of macroscopic quantum tunnelling in Bi2212 intrinsic Josephsonjunctions

The quantum dynamics of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions (IJJs) is studied based on escape rate measurements. The saturationsobserved in the escape temperature and in the width of the switching current below 0.5 K(= T*) indicate the transition of the switching mechanism from thermal activation to macroscopicquantum tunnelling. It is shown that the switching properties are consistentlyexplained in terms of the underdamped Josephson junction with a quality factor of70 ± 20 in spite of possible damping due to the nodal quasiparticles of d-wave superconductivity.The present result gives the upper limit of the damping of IJJs.

Macroscopic quantum dynamics ofπjunctions with ferromagnetic insulators

We theoretically investigate the macroscopic quantum dynamics of a pi junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a pi-junction qubit (a S-FI-S superconducting quantum interference device ring) is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency. We also investigate macroscopic quantum tunneling (MQT) in current-biased S-FI-S pi junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small.

Critical velocities in superfluids and the nucleation of vortices

The problem of critical velocities in superfluids, that is the comprehension of superfluidity breakdown by flow, has been long standing. One difficulty stems from the existence of several breakdown mechanisms. A major advance has come from the observation of single 2 π phase slips, which arise from the nucleation of quantised vortices, that is, their creation ex nihilo. The statistical properties of the nucleation process in both the thermal regime and the quantum regime are identified and analysed: vortex nucleation provides a well-documented case of macroscopic quantum tunnelling (MQT). In particular, a close scrutiny of the experimental data obtained on ultra-pure 4He reveals the influence of damping on tunnelling, a rare occurrence where the effect of the environment on MQT can be studied. To cite this article: É. Varoquaux, C. R. Physique 7 (2006).

Quantum tunneling of magnetization in dipolar spin-1 condensates under external fields

We study the macroscopic quantum tunneling of magnetization of the $F=1$ spinor condensate interacting through dipole-dipole interaction with an external magnetic field applied along the longitudinal or transverse direction. We show that the ground state energy and the effective magnetic moment of the system exhibit an interesting macroscopic quantum oscillation phenomenon originating from the oscillating dependence of thermodynamic properties of the system on the vacuum angle. Tunneling between two degenerate minima are analyzed by means of an effective potential method and the periodic instanton method.

Strong cotunneling suppression in a single-electron transistor with granulated metal film island

Macroscopic quantum tunneling (cotunneling) is a major source of errors in single-electron devices that require the transfer of a precise number of electrons. The authors present a single-electron transistor (SET) where the suppression of cotunneling is achieved by using a granulated metal film as the material of the SET island. In this device a characteristic charging energy is defined by the Al∕AlOx junctions while the cotunneling is suppressed by electron scattering in the granulated metal island. The authors discuss possible applications of this solution for single-electron latches and pumps.

Real-time approach to tunnelling in open quantum systems: decoherence and anomalous diffusion

Macroscopic quantum tunneling is described using the master equation for the reduced Wigner function of an open quantum system at zero temperature. Our model consists of a particle trapped in a cubic potential interacting with an environment characterized by dissipative and normal and anomalous diffusion coefficients. A representation based on the energy eigenfunctions of the isolated system, i.e. the system uncoupled to the environment, is used to write the reduced Wigner function, and the master equation becomes simpler in that representation. The energy eigenfunctions computed in a WKB approximation incorporate the tunneling effect of the isolated system and the effect of the environment is described by an equation that it is in many ways similar to a Fokker-Planck equation. Decoherence is easily identified from the master equation and we find that when the decoherence time is much shorter than the tunneling time the master equation can be approximated by a Kramers like equation describing thermal activation due to the zero point fluctuations of the quantum environment. The effect of anomalous diffusion can be dealt with perturbatively and its overall effect is to inhibit tunneling.

Evidence for Macroscopic Quantum Tunneling of Phase Slips in Long One-Dimensional Superconducting Al Wires

Quantum phase slips have received much attention due to their relevance to superfluids in reduced dimensions and to models of cosmic string production in the early universe. Their establishment in one-dimensional superconductors has remained controversial. Here we study the nonlinear current-voltage characteristics and linear resistance in long superconducting Al wires with lateral dimensions approximately 5 nm. We find that, in a magnetic field and at temperatures well below the superconducting transition, the observed behaviors can be described by the nonclassical, macroscopic quantum tunneling of phase slips, and are inconsistent with the thermal activation of phase slips.

Resonance Phenomena in the Macroscopic Quantum Tunnelling

In this paper we present a theoretical approach to describe the quantum behaviour of a macroscopic system interacting with an external field at frequencies close to resonant condition. Moreover we apply our results to simulate resonant phenomena in rf SQUIDs, whose parameters lie in the range typically used in the experiments.

Enhanced Macroscopic Quantum Tunneling inBi2Sr2CaCu2O8+δIntrinsic Josephson-Junction Stacks

We have investigated macroscopic quantum tunneling in Bi(2)Sr(2)CaCu(2)O(8 + delta) intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of Nu junctions is about Nu(2) times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of the macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.