Quantum and Classical Dissipative Effects on Tunnelling in Quantum Hall Bilayers

We discuss the interplay between transport and dissipation in quantum Hall bilayers. We show that quantum effects are relevant in the pseudospin picture of these systems, leading either to direct tunnelling currents or to quantum dissipative processes that damp oscillations around the ground state. These quantum effects have their origins in resonances of the classical spin system.Comment: 12 pages. Minor changes from v

Observing controlled state collapse in a single mechanical oscillator via a direct probe of energy variance

Due to their central role in our classical intuition of the physical world and their potential for interacting with the gravitational field, mechanical degrees of freedom are of special interest in testing the nonclassical predictions of quantum theory at ever larger scales. The projection postulate of quantum theory predicts that, for certain types of measurements, continuously measuring a system induces a stochastic collapse of the state of the system toward a random eigenstate. Here we propose an optomechanical scheme to observe this fundamental effect in a vibrational mode of a mechanical membrane. The observation in the scheme is direct (it is not inferred via an a priori assumption of the projection postulate for the mechanical mode) and is made possible through an in situ probe of the mechanical energy variance. In the scheme, quantum theory predicts that a steady state is reached as the measurement-induced collapse is counteracted by dissipation to the unmonitored environment. Numerical simulations show this to result in a monotonic decrease in the time-averaged energy variance as the ratio of continuous measurement strength to dissipation is increased. The measurement strength in the proposed scheme is tunable in situ, and the behavior predicted by the simulations therefore implies a way to verifiably control the time-averaged variance of a mechanical wave function over the course of a single quantum trajectory. The scheme's ability to directly probe the energy variance of the mechanical mode may also enable further investigations of the effects on the mechanical state of coupling the mechanical mode to other quantum systems

Low- and high-frequency noise from coherent two-level systems

Recent experiments indicate a connection between the low- and high-frequency noise affecting superconducting quantum systems. We explore the possibilities that both noises can be produced by one ensemble of microscopic modes, made up, e.g., by sufficiently coherent two-level systems (TLS). This implies a relation between the noise power in different frequency domains, which depends on the distribution of the parameters of the TLSs. We show that a distribution, natural for tunneling TLSs, with a log-uniform distribution in the tunnel splitting and linear distribution in the bias, accounts for experimental observations.Comment: minor corrections, references adde

Water on Pt(111): the importance of proton disorder

The structure of a water adlayer on Pt(111) surface is investigated by extensive first principle calculations. Only allowing for proton disorder the ground state energy can be found. This results from an interplay between water/metal chemical bonding and the hydrogen bonding of the water network. The resulting short O-Pt distance accounts for experimental evidences. The novelty of these results shed a new light on relevant aspects of water-metal interaction.Comment: 10 pages 4 figures (color

Dynamics of a Josephson Array in a Resonant Cavity

We derive dynamical equations for a Josephson array coupled to a resonant cavity by applying the Heisenberg equations of motion to a model Hamiltonian described by us earlier [Phys. Rev. B {\bf 63}, 144522 (2001); Phys. Rev. B {\bf 64}, 179902 (E)]. By means of a canonical transformation, we also show that, in the absence of an applied current and dissipation, our model reduces to one described by Shnirman {\it et al} [Phys. Rev. Lett. {\bf 79}, 2371 (1997)] for coupled qubits, and that it corresponds to a capacitive coupling between the array and the cavity mode. From extensive numerical solutions of the model in one dimension, we find that the array locks into a coherent, periodic state above a critical number of active junctions, that the current-voltage characteristics of the array have self-induced resonant steps (SIRS's), that when $N_a$ active junctions are synchronized on a SIRS, the energy emitted into the resonant cavity is quadratic in $N_a$, and that when a fixed number of junctions is biased on a SIRS, the energy is linear in the input power. All these results are in agreement with recent experiments. By choosing the initial conditions carefully, we can drive the array into any of a variety of different integer SIRS's. We tentatively identify terms in the equations of motion which give rise to both the SIRS's and the coherence threshold. We also find higher-order integer SIRS's and fractional SIRS's in some simulations. We conclude that a resonant cavity can produce threshold behavior and SIRS's even in a one-dimensional array with appropriate experimental parameters, and that the experimental data, including the coherent emission, can be understood from classical equations of motion.Comment: 15 pages, 10 eps figures, submitted to Phys. Rev.