Macroscopic quantum tunneling in d-wave YBa2Cu3O7-delta Josephson junctions

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa(2)Cu(3)O(7-delta) grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.     

Zener enhancement of quantum tunneling in a two-level superconducting circuit

We have investigated the macroscopic quantum tunneling (MQT) of the phase across a Josephson junction embedded in a superconducting circuit. This system is equivalent to a spin 1/2 particle in a potential energy well. The MQT escape rate of such a particle was recently predicted to be strongly modified when a crossing of its inner Zeeman levels occurs while tunneling. In this regime, we observe a significant enhancement of the MQT rate and compare it to theory.     

Effects of capacitive coupling on the escape rate in intrinsic Josephson junction stacks

We formulate a macroscopic quantum theory that can describe the macroscopic quantum tunneling (MQT) in intrinsic Josephson junctions (IJJs). The capacitively coupled IJJ model is quantized in terms of the canonical quantization method. The multi-junction effect for the MQT to the first resistive branch is clarified. It is shown that the escape rate is greatly enhanced by the capacitive coupling between junctions.     

Quantitative study of macroscopic quantum tunneling in a dc SQUID: a system with two degrees of freedom

To test whether the theory of macroscopic quantum tunneling (MQT) is applicable to systems with 2 degrees of freedom, we experimentally investigated the switching current distribution of a dc SQUID. Using sample parameters determined from measurements at T=4.2 K, we are able to make quantitative comparison to the theories from 8 mK to 4.2 K. The excellent agreement between the data and the MQT theory demonstrates that tunneling from the zero-voltage state of the dc SQUID is well described by the quantum mechanics.     

Macroscopic quantum tunneling induced by a spontaneous field in intrinsic Josephson junctions

We theoretically study macroscopic quantum tunneling (MQT) in capacitively coupled Josephson junctions in the case that one of the junctions is in the finite voltage state. We find that the system can be mapped into a one dimensional model with a spontaneous periodic perturbation and calculate the MQT rate by using the time-dependent WKB method. Then the MQT rate is found to be resonantly enhanced and the enhancement of MQT rate is found even off the resonant point.     

Quantum phase diffusion in a small underdamped Josephson junction

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (～0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.     

本征约瑟夫森结跳变电流分布的量子修正

在约瑟夫森结跳变电流统计分布的理论拟合过程中,通常考虑的是宏观量子隧穿与热激活这两种过程. 对Bi₂Sr₂CaCu₂O_8+δ表面本征约瑟夫森结的结果分析表明,在宏观量子隧穿与热激活的交界区域内,若考虑量子修正能使实验曲线与理论曲线符合得更好. 这种较为完整的拟合方法,对研究本征约瑟夫森器件中的宏观量子现象及其在超导量子比特中的应用具有积极的意义. 关键词： 约瑟夫森结 跳变电流分布 量子修正     

Observation of macroscopic quantum tunneling in a single Bi2Sr2CaCu2O8+delta surface intrinsic Josephson junction

We report on the first unambiguous observation of macroscopic quantum tunneling (MQT) in a single submicron Bi(2)Sr(2)CaCu(2)O(8+delta) surface intrinsic Josephson junction (IJJ) by measuring its temperature-dependent switching current distribution. All relevant junction parameters were determined in situ in the classical regime and were used to predict the behavior of the IJJ in the quantum regime via MQT theory. Experimental results agree quantitatively with the theoretical predictions, thus confirming the MQT picture. Furthermore, the data also indicate that the surface IJJ, where the current flows along the c axis of the crystal, has the conventional sinphi current-phase relationship.     

Evidence of two-dimensional macroscopic quantum tunneling of a current-biased dc SQUID

The escape probability out of the superconducting state of a hysteretic dc SQUID has been measured at different values of the applied magnetic flux. At low temperatures, the escape current and the width of the probability distribution are temperature independent but they depend on flux. Experimental results do not fit the usual one-dimensional macroscopic quantum tunneling (MQT) law but are perfectly accounted for by the two-dimensional MQT behavior as we propose here. Near zero flux, our data confirms the recent MQT observation in a dc SQUID [Phys. Rev. Lett. 89, 98301 (2002)]].     

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.     

Macroscopic quantum tunneling in differently loaded Josephson junctions

In relation to the problem of quantum measurement, macroscopic quantum tunneling (MQT) in Josephson junctions is one of the most investigated topics. Although many theoretical studies have been devoted to this argument, the agreement on a single theoretical model has not yet been reached. The purpose of the present work is to analyze the load “seen” by the junction, in order to evaluate the contribution of the dissipation that modifies the decay-rate of the metastable state of the junction. The present work reports some theoretical results in relation to different kinds of loading systems, namely, the cases of capacitive, inductive and purely resistive load. The resistive load is considered also in relation to an experimental test.     

Metastable state and macroscopic quantum tunneling of binary mixtures

We provide a mixing model to explore the metastable state and the macroscopic quantum tunneling (MQT) of binary mixtures. For , we first observe the two condensates form the symmetry-breaking state (SBS) and then suddenly transfer to the symmetry-preserving state (SPS) through the MQT. The SBS is shown to be the metastable state in our system. We find the MQT does not spontaneously arise. The inducement mechanism is the damping but not the excitations. The damping mechanism can also control the lifetime and the tunneling decay rate of the SBS. Finally, we further present the origin of these phenomena by examining the energy of the system.     

Magnetic long-range order induced by quantum relaxation in single-molecule magnets

Can magnetic interactions between single-molecule magnets (SMMs) in a crystal establish long-range magnetic order at low temperatures deep in the quantum regime, where the only electron spin fluctuations are due to incoherent magnetic quantum tunneling (MQT)? Put inversely: can MQT provide the temperature dependent fluctuations needed to destroy the ordered state above some finite T(c), although it should basically itself be a T-independent process? Our experiments on two novel Mn4 SMMs provide a positive answer to the above, showing at the same time that MQT in the SMMs has to involve spin-lattice coupling at a relaxation rate equaling that predicted and observed recently for nuclear-spin-mediated quantum relaxation.     

Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8

Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe nuclear spins leads to a huge enhancement of the specific heat below 1 K, ascribed to a strong increase in the spin-lattice relaxation rate gamma arising from incoherent, nuclear-spin-mediated magnetic quantum tunneling (MQT) in the ground doublet. Since gamma is found comparable to the expected tunneling rate, the MQT process has to be inelastic. A model for the coupling of the tunneling spins to the lattice is presented. Under transverse field, a crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.     

Dynamics of a three level atom interacting with a detuned SU (1,1) quantum system

We study the stability of attractive atomic Bose-Einstein condensate and the macroscopic quantum many-body tunneling (MQT) in the anharmonic trap. We utilize correlated two-body basis function which keeps all possible two-body correlations. The anharmonic parameter (λ) is slowly tuned from harmonic to anharmonic. For each choice of λ the many-body equation is solved adiabatically. The use of the van der Waals interaction gives realistic picture which substantially differs from the mean-field results. For weak anharmonicity, we observe that the attractive condensate gains stability with larger number of bosons compared to that in the pure harmonic trap. The transition from resonances to bound states with weak anharmonicity also differs significantly from the earlier study of [N. Moiseyev, L.D. Carr, B.A. Malomed, Y.B. Band, J. Phys. B 37, L193 (2004)]. We also study the tunneling of the metastable condensate very close to the critical number N _cr of collapse and observe that near collapse the MQT is the dominant decay mechanism compared to the two-body and three-body loss rate. We also observe the power law behavior in MQT near the critical point. The results for pure harmonic trap are in agreement with mean-field results. However, we fail to retrieve the power law behavior in anharmonic trap although MQT is still the dominant decay mechanism.     

Electrically tunable macroscopic quantum tunneling in a graphene-based Josephson junction

Stochastic switching-current distribution in a graphene-based Josephson junction exhibits a crossover from the classical to quantum regime, revealing the macroscopic quantum tunneling of a Josephson phase particle at low temperatures. Microwave spectroscopy measurements indicate a multiphoton absorption process occurring via discrete energy levels in washboard potential well. The crossover temperature for macroscopic quantum tunneling and the quantized level spacing are controlled with the gate voltage, implying its potential application to gate-tunable superconducting quantum bits.     

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 2π 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).     

Macroscopic resonant tunneling in an rf-SQUID flux qubit

We have observed the macroscopic resonant tunneling of magnetic flux between macroscopically distinct quantum states in a superconducting flux qubit.The dependences of the macroscopic resonant tunneling on the barrier height of the potential well,the flux bias and the initial state are investigated.Detailed measurements of the tunneling rate as a function of the flux bias reveal the feature of the quantum noise in the superconducting flux qubit.