Quantum metamaterials: Electromagnetic waves in a Josephson qubit line

We consider the propagation of a classical electromagnetic wave through a transmission line, formed by identical superconducting charge qubits inside a superconducting resonator. Since the qubits can be in a coherent superposition of quantum states, we show that such a system demonstrates interesting new effects, such as a ``breathing'' photonic crystal with an oscillating bandgap, and a ``quantum Archimedean screw'' that transports, at an arbitrary controlled velocity, Josephson plasma waves through the transmission line. The key ingredient of these effects is that the optical properties of the Josephson transmission line are controlled by the quantum coherent state of the qubits.Comment: References adde

Noise enhanced performance of adiabatic quantum computing by lifting degeneracies

We investigate the symmetry breaking role of noise in adiabatic quantum computing using the example of the CNOT gate. In particular, we analyse situations where the choice of initial configuration leads to symmetries in the Hamiltonian and degeneracies in the spectrum. We show that, in these situations, there exists an optimal level of noise that maximises the success probability and the fidelity of the final state. The effects of an artificial noise source with a time-dependent amplitude are also explored and it is found that such a scheme would offer a considerable performance enhancement.Comment: 12 pages and 4 figures in preprint format. References in article corrected and journal reference adde

Thermal non-equilibrium effects in quantum reflection

We show that the quantum reflection coefficient of ultracold heavy atoms scattering off a dielectric surface can be tuned in a wide range by suitable choice of surface and environment temperatures. This effect results from a temperature dependent long-range repulsive part of the van der Waals-Casimir-Polder-Lifshitz atom-surface interaction potential

Directed motion of domain walls in biaxial ferromagnets under the influence of periodic external magnetic fields

Directed motion of domain walls (DWs) in a classical biaxial ferromagnet placed under the influence of periodic unbiased external magnetic fields is investigated. Using the symmetry approach developed in this article the necessary conditions for the directed DW motion are found. This motion turns out to be possible if the magnetic field is applied along the most easy axis. The symmetry approach prohibits the directed DW motion if the magnetic field is applied along any of the hard axes. With the help of the soliton perturbation theory and numerical simulations, the average DW velocity as a function of different system parameters such as damping constant, amplitude, and frequency of the external field, is computed.Comment: Added references, corrected typos, extended introductio

Resonance effects due to the excitation of surface Josephson plasma waves in layered superconductors

We analytically examine the excitation of surface Josephson plasma waves (SJPWs) in periodically-modulated layered superconductors. We show that the absorption of the incident electromagnetic wave can be substantially increased, for certain incident angles, due to the resonance excitation of SJPWs. The absorption increase is accompanied by the decrease of the specular reflection. Moreover, we find the physical conditions guaranteeing the total absorption (and total suppression of the specular reflection). These conditions can be realized for Bi2212 superconductor films.Comment: 17 pages, 3 figure

Quantum electromechanics: Quantum tunneling near resonance and qubits from buckling nanobars

Analyzing recent experimental results, we find similar behaviors and a deep analogy between three-junction superconducting qubits and suspended carbon nanotubes. When these different systems are ac-driven near their resonances, the resonance single-peak, observed at weak driving, splits into two sub-peaks (Fig. 1) when the driving increases. This unusual behavior can be explained by considering quantum tunneling in a double well potential for both systems. Inspired by these experiments, we propose a mechanical qubit based on buckling nanobars--a NEMS so small as to be quantum coherent. To establish buckling nanobars as legitimate candidates for qubits, we calculate the effective buckling potential that produces the two-level system and identify the tunnel coupling between the two local states. We propose different designs of nanomechanical qubits and describe how they can be manipulated. Also, we outline possible decoherence channels and detection schemes. A comparison between nanobars and well studied superconducting qubits suggests several future experiments on quantum electromechanics.Comment: 6 pages, 3 figures, 1 tabl

Transport and localization in periodic and disordered graphene superlattices

We study charge transport in one-dimensional graphene superlattices created by applying layered periodic and disordered potentials. It is shown that the transport and spectral properties of such structures are strongly anisotropic. In the direction perpendicular to the layers, the eigenstates in a disordered sample are delocalized for all energies and provide a minimal non-zero conductivity, which cannot be destroyed by disorder, no matter how strong this is. However, along with extended states, there exist discrete sets of angles and energies with exponentially localized eigenfunctions (disorder-induced resonances). It is shown that, depending on the type of the unperturbed system, the disorder could either suppress or enhance the transmission. Most remarkable properties of the transmission have been found in graphene systems built of alternating p-n and n-p junctions. This transmission has anomalously narrow angular spectrum and, surprisingly, in some range of directions it is practically independent of the amplitude of fluctuations of the potential. Owing to these features, such samples could be used as building blocks in tunable electronic circuits. To better understand the physical implications of the results presented here, most of our results have been contrasted with those for analogous wave systems. Along with similarities, a number of quite surprising differences have been found.Comment: 10 page

Nonuniform Self-Organized Dynamical States in Superconductors with Periodic Pinning

We consider magnetic flux moving in superconductors with periodic pinning arrays. We show that sample heating by moving vortices produces negative differential resistivity (NDR) of both N and S type (i.e., N- and S-shaped) in the voltage-current characteristic (VI curve). The uniform flux flow state is unstable in the NDR region of the VI curve. Domain structures appear during the NDR part of the VI curve of an N type, while a filamentary instability is observed for the NDR of an S type. The simultaneous existence of the NDR of both types gives rise to the appearance of striking self-organized (both stationary and non-stationary) two-dimensional dynamical structures.Comment: 4 pages, 2 figure

Electrodynamics of Abrikosov vortices: the Field Theoretical Formulation

Electrodynamic phenomena related to vortices in superconductors have been studied since their prediction by Abrikosov, and seem to hold no fundamental mysteries. However, most of the effects are treated separately, with no guiding principle. We demonstrate that the relativistic vortex worldsheet in spacetime is the object that naturally conveys all electric and magnetic information, for which we obtain simple and concise equations. Breaking Lorentz invariance leads to down-to-earth Abrikosov vortices, and special limits of these equations include for instance dynamic Meissner screening and the AC Josephson relation. On a deeper level, we explore the electrodynamics of two-form sources in the absence of electric monopoles, in which the electromagnetic field strength itself acquires the characteristics of a gauge field. This novel framework leaves room for unexpected surprises.Comment: LaTeX, 23 pages, 5 figure

Electron localization in sound absorption oscillations in the quantum Hall effect regime

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/AlGaAs heterostructure (with two-dimensional electron mobility $\mu= 1.3\times 10^5 cm^2/V\cdot s)$ at T=4.2K) via a small gap has been investigated experimentally as a function of the frequency of the wave, the width of the vacuum gap, the magnetic field, and the temperature. The magnetic field and frequency dependencies of the high-frequency conductivity (in the region 30-210 MHz) are calculated and analyzed. The experimental results can be explained if it assumed that there exists a fluctuation potential in which current carrier localization occurs. The absorption of the surface acoustic waves in an interaction with two-dimensional electrons localized in the energy "tails" of Landau levels is discussed.Comment: RevTeX 6 pages+6 EPS pic