Quantum tunneling at zero temperature in the strong friction regime

In the large damping limit we derive a Fokker-Planck equation in configuration space (the so-called Smoluchowski equation) describing a Brownian particle immersed into a thermal environment and subjected to a nonlinear external force. We quantize this stochastic system and survey the problem of escape over a double-well potential barrier. Our finding is that the quantum Kramers rate does not depend on the friction coefficient at low temperatures; i.e., we predict a superfluidity phenomenon in overdamped open systems. Moreover, at zero temperature we show that the quantum escape rate does not vanish in the strong friction regime. This result, therefore, is in contrast with the work by Ankerhold et al. [Phys. Rev. Lett. 87, 086802 (2001)]] in which no quantum tunneling is predicted at zero temperature.     

Quantum tunneling of ultracold atoms in optical traps

We review our theoretical advances in quantum tunneling of BoseEinstein condensates in optical traps and in microcavities. By employing a real physical system, the frequencies of the pseudo Goldstone modes in different phases between two optical traps are studied respectivdy, which are tile crucial feature of the non-Abelian Joseptmon effect. When the optical lattices are under gravity, we investigate the quantum tummling in the ＂Wannier-Stark localization＂ regime and ＂Lan（lau Zener tunneling＂ regime. We finally get the total decay rate and the rate is valid over the entire range of temperatures. At high temperatures, we show how the decay rate reduces to the appropriate results for the classical thermal activation. At hltermediate temperatures, the results of tile total decay rate are consistent with the thermally assisted tunneling. At low temperatures, we obtain the pure quantmn tunneling ultimately. And we study the alternating-current and direct-current （ac and de） photonic 3osephson effects in two weakly linked microcavities containing ultracold two-level atones, which allows for direct observation of the effects. This enables new investigations of the effect of maw-body physics in strongly coupled atom-cavity systems and provides a strategy for constructing novel interference devices of coherent photons. In addition, we propose the experimental protocols to observe these quantmn tunneling of Bose- Einstein condensates.     

Effects of structural disorders on sequential tunneling in multiple quantum wells

Based on an isotropic random distribution model, the effects of structural disorders embedded in the barriers on the sequential electron tunneling in multiple quantum wells were studied at low temperatures. By using a sequential tunneling model [Stievenard et al., Appl. Phys. Lett. 61 (1992) 1582], the transmission coefficient through a single barrier was calculated using a finite-difference method and averaged over random configurations of disorders. To compute the tunneling current, a self-consistent calculation for the electronic states was performed, including the Hartree and exchange interactions and non-parabolic energy dispersion. Both disorder-assisted and disorder-impeded electron tunneling phenomena were found as a function of the activation energy. The effects of electric field, barrier width, and temperature were also studied. The predicted resonant disorder-assisted electron tunneling should be large enough to be observable at low temperatures in an experiment.     

Tunneling spectroscopy of localized states near the quantum Hall edge

In this paper, we discuss the experimental results of M. Grayson et al. on tunneling I–V characteristics of the quantum Hall edge. We suggest a two-step tunneling mechanism involving localized electron states near the edge, which might account for the discrepancy between the experimental data and the predictions of the chiral Luttinger liquid theory of the quantum Hall edge.     

Superconductor-insulator transition in a two-dimensional array of resistively shunted small josephson junctions

Using a scattering technique based on a parametrized linear combination of atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of multiwall carbon nanotubes. We find that interwall interactions not only block some of the quantum conductance channels, but also redistribute the current nonuniformly over individual tubes across the structure. Our results provide a natural explanation for the unexpected integer and noninteger conductance values reported for multiwall nanotubes by Stefan Frank et al. [Stefan Frank et al., Science 280, 1744 (1998)].     

Quantum Tunneling from Apparent Horizon of?Rainbow-FRW Universe

The quantum tunneling from the apparent horizon of rainbow-FRW universe is studied in this paper. We apply the semi-classical approximation, which is put forward by Parikh and Wilczek et al., to research on the scalar field particles tunneling from the apparent horizon of the rainbow-FRW universe, and then use the spin 1/2 Fermions tunneling theory, which brought forward by Kerner and Mann firstly, to research on the Fermions Hawking radiation via semi-classical approximation. Finally, we discuss the meanings of the quantum effect via Finsler geometry.     

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.     

A proposal for the implementation of a quantum phase gate in a five-level atomic medium

The interaction of a five-level atomic system involving electromagnetically induced transparency with four light fields is investigated. Two different operating regimes are considered, semi-classical and quantized, where two light fields are treated as quantum. In the semi-classical regime, the dispersive properties of the scheme are analyzed in detail, and the conditions leading to group velocity matching for two of the light fields are identified. In the quantized regime, the two quantum fields undergo cross-phase modulation induced by electromagnetically induced transparency. Based on this, a recent proposal [C. Ottaviani et al., Phys. Rev. A 73, 010301 (R) (2006)] for a two-qubit quantum phase gate for traveling single-photon qubits is examined. An experimental scheme for verification of QPG operation is proposed.     

Coherent versus sequential electron tunneling in quantum dots

Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb-blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels.     

Experimental study of a Back Action Evading device for continuos measurements on a macroscopic harmonic oscillator at the quantum limit level

The Back Action Evading technique is a particular kind of quantum non demolition measurement, first proposed by Caves et al. in 1980 [3]. We present an experimental study to implement the Back Action Evading measurement scheme in monitoring the amplitude of an harmonic oscillator excited by a classical force. Results showing the agreement of our theoretical model with the experimental behaviour of our apparatus in the classical regime are presented. We discuss also the optimization of the performance of our set-up, which should allow to monitor our oscillator in quantum regime even below the standard quantum limit level. Received: 22 March 1996     

Prediction of ferromagnetic correlations in coupled double-level quantum dots

Numerical results for transport properties of two coupled double-level quantum dots (QDs) strongly suggest that under appropriate conditions the dots develop a novel ferromagnetic (FM) correlation at quarter filling (one electron per dot). In the strong coupling regime (Coulomb repulsion larger than electron hopping) and with interdot tunneling larger than tunneling to the leads, an S=1 Kondo resonance develops in the density of states, leading to a peak in the conductance. A qualitative "phase diagram," incorporating the new FM phase, is presented. In addition, the necessary conditions for the FM regime are less restrictive than naively believed, leading to its possible experimental observation in real QDs.     

Reconstruction of fractional quantum Hall edges

We study the interplay of electron-electron interaction, confining potential and effects of finite temperature at the edge of a quantum Hall liquid. Our exact diagonalization calculation indicates that edge reconstruction occurs in the fractional quantum Hall regime for a variety of confining potential, including ones that correspond to a "sharp" edge. Our finite temperature Hartree-Fock calculation for integer quantum Hall edges indicates that reconstruction is suppressed above a certain temperature. We discuss the implication of our results on recent edge tunneling and microwave absorption experiments.     

Wave-particle dualism and the interpretation of quantum mechanics

The realist interpretations of quantum theory, proposed by de Broglie and by Bohm, are re-examined and their differences, especially concerning many-particle systems and the relativistic regime, are explored. The impact of the recently proposed experiments of Vigier et al. and of Ghose et al. on the debate about the interpretation of quantum mechanics is discussed. An indication of how de Broglie and Bohm would account for these experimental results is given.     

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.     

Physics Without Physics

David Finkelstein was very fond of the new information-theoretic paradigm of physics advocated by John Archibald Wheeler and Richard Feynman. Only recently, however, the paradigm has concretely shown its full power, with the derivation of quantum theory (Chiribella et al., Phys. Rev. A 84:012311, 2011; D’Ariano et al., 2017) and of free quantum field theory (D’Ariano and Perinotti, Phys. Rev. A 90:062106, 2014; Bisio et al., Phys. Rev. A 88:032301, 2013; Bisio et al., Ann. Phys. 354:244, 2015; Bisio et al., Ann. Phys. 368:177, 2016) from informational principles. The paradigm has opened for the first time the possibility of avoiding physical primitives in the axioms of the physical theory, allowing a re-foundation of the whole physics over logically solid grounds. In addition to such methodological value, the new information-theoretic derivation of quantum field theory is particularly interesting for establishing a theoretical framework for quantum gravity, with the idea of obtaining gravity itself as emergent from the quantum information processing, as also suggested by the role played by information in the holographic principle (Susskind, J. Math. Phys. 36:6377, 1995; Bousso, Rev. Mod. Phys. 74:825, 2002). In this paper I review how free quantum field theory is derived without using mechanical primitives, including space-time, special relativity, Hamiltonians, and quantization rules. The theory is simply provided by the simplest quantum algorithm encompassing a countable set of quantum systems whose network of interactions satisfies the three following simple principles: homogeneity, locality, and isotropy. The inherent discrete nature of the informational derivation leads to an extension of quantum field theory in terms of a quantum cellular automata and quantum walks. A simple heuristic argument sets the scale to the Planck one, and the currently observed regime where discreteness is not visible is the so-called “relativistic regime” of small wavevectors, which holds for all energies ever tested (and even much larger), where the usual free quantum field theory is perfectly recovered. In the present quantum discrete theory Einstein relativity principle can be restated without using space-time in terms of invariance of the eigenvalue equation of the automaton/walk under change of representations. Distortions of the Poincaré group emerge at the Planck scale, whereas special relativity is perfectly recovered in the relativistic regime. Discreteness, on the other hand, has some plus compared to the continuum theory: 1) it contains it as a special regime; 2) it leads to some additional features with GR flavor: the existence of an upper bound for the particle mass (with physical interpretation as the Planck mass), and a global De Sitter invariance; 3) it provides its own physical standards for space, time, and mass within a purely mathematical adimensional context. The paper ends with the future perspectives of this project, and with an Appendix containing biographic notes about my friendship with David Finkelstein, to whom this paper is dedicated.     

Quantum spin Hall effect in inverted InAs/GaSb quantum wells

We review the recent experimental progress towards observing quantum spin Hall effect in inverted InAs/GaSb quantum wells (QWs). Low temperature transport measurements in the hybridization gap show bulk conductivity of a non-trivial origin, while the length and width dependence of conductance in this regime show strong evidence for the existence of helical edge modes proposed by Liu et al. [Phys. Rev. Lett., 2008, 100: 236601]. Surprisingly, edge modes persist in spite of comparable bulk conduction and show only weak dependence on magnetic field. We elucidate that seeming independence of edge on bulk transport comes due to the disparity in Fermi-wave vectors between the bulk and the edge, leading to a total internal reflection of the edge modes.     

Aharonov-Bohm Effect and High-Velocity Estimates of Solutions to the Schr?dinger Equation

The Aharonov-Bohm effect is a fundamental issue in physics that has been extensively studied in the literature and is discussed in most of the textbooks in quantum mechanics. The issues at stake are what are the fundamental electromagnetic quantities in quantum physics, if magnetic fields can act at a distance on charged particles and if the magnetic potentials have a real physical significance. The Aharonov-Bohm effect is a very controversial issue. From the experimental side the issues were settled by the remarkable experiments of Tonomura et?al. (Phys Rev Lett 48:1443?C1446, 1982; Phys Rev Lett 56:792?C795, 1986) with toroidal magnets that gave a strong experimental evidence of the physical existence of the Aharonov-Bohm effect, and by the recent experiment of Caprez et?al. (Phys Rev Lett 99:210401, 2007) that shows that the results of the Tonomura et?al. experiments can not be explained by the action of a force. Aharonov and Bohm (Phys Rev 115:485-491, 1959) proposed an Ansatz for the solution to the Schr?dinger equation in simply connected regions of space where there are no electromagnetic fields. It consists of multiplying the free evolution by the Dirac magnetic factor. The Aharonov-Bohm Ansatz predicts the results of the experiments of Tonomura et?al. and of Caprez et?al. Recently in Ballesteros and Weder (Math Phys 50:122108, 2009) we gave the first rigorous proof that the Aharonov-Bohm Ansatz is a good approximation to the exact solution for toroidal magnets under the conditions of the experiments of Tonomura et?al. We provided a rigorous, simple, quantitative, error bound for the difference in norm between the exact solution and the Aharonov-Bohm Ansatz. In this paper we prove that these results do not depend on the particular geometry of the magnets and on the velocities of the incoming electrons used on the experiments, and on the gaussian shape of the wave packets used to obtain our quantitative error bound. We consider a general class of magnets that are a finite union of handlebodies. Each handlebody is diffeomorphic to a torus or a ball, and some of them can be patched though the boundary. We formulate the Aharonov-Bohm Ansatz that is appropriate to this general case and we prove that the exact solution to the Schr?dinger equation is given by the Aharonov-Bohm Ansatz up to an error bound in norm that is uniform in time and that decays as a constant divided by v ^?? , 0 < ?? <?1, with v the velocity. The results of Tonomura et?al., of Caprez et?al., our previous results and the results of this paper give a firm experimental and theoretical basis to the existence of the Aharonov-Bohm effect and to its quantum nature. Namely, that magnetic fields act at a distance on charged particles, and that this action at a distance is carried by the circulation of the magnetic potential which gives a real physical significance to magnetic potentials.     

Quantum critical Kondo quasiparticles probed by ESR in β-YbAlB₄

Electron spin resonance (ESR) can probe conduction electrons (CE) and local moment (LM) spin systems in different materials. A CE spin resonance (CESR) is observed in metallic systems based on light elements or with enhanced Pauli susceptibility. LM ESR can be seen in compounds with paramagnetic ions and localized d or f electrons. Here we report a remarkable and unprecedented ESR signal in the heavy-fermion superconductor β-YbAlB? [S. Nakatsuji et al., Nature Phys. 4, 603 (2008)] which behaves as a CESR at high temperatures and acquires characteristics of the Yb3? LM ESR at low temperature. This dual behavior strikes as an in situ unique observation of the Kondo quasiparticles in a quantum critical regime. The proximity to a quantum critical point may favor the appearance of this dual character of the ESR signal in β-YbAlB?.     

Possible manifestations of the quantum effect (Tunneling) in elementary events in the fracture kinetics of polymers

An elementary event in the kinetics of fracture of polymers, i.e., breaking of a stressed skeletal bond in a chain molecule, has been simulated by the decay of a loaded quantum anharmonic oscillator. The probability and the average time of expectation of the escape of a particle from the potential well in the Morse potential under the action of a tensile force have been calculated over a wide range of temperatures. It has been demonstrated that the escape of the particle occurs predominantly through the tunneling mechanism at low and medium temperatures and through a combination of the tunneling (under-barrier) and over-barrier (thermal-fluctuation) mechanisms with comparable contributions at high temperatures. The calculations have revealed that the participation of the tunneling mechanism in the kinetics of fracture of polymers manifests itself in a low-temperature athermal plateau in the temperature dependence of the breaking strength. A comparison between the calculated and experimental temperature dependences of the breaking strength for the oriented polymer polycaproamide has shown that the calculated and experimental results are in qualitative and quantitative agreement, which allows the conclusion that the tunneling mechanism can contribute to the fracture of polymers.