Brownian motion: a case of temperature fluctuations

A diffusion process of a Brownian particle in a medium of temperature $T$ is re-considered. We assume that temperature of the medium fluctuates around its mean value. The velocity probability distribution is obtained. It is shown that the stationary state is not a thermodynamic equilibrium state described by the Maxwell distribution. Instead a nonequilibrium state is produced by temperature fluctuations.Comment: accepted for publication in Acta Physica Polonica

Geometric phase as a determinant of a qubit--environment coupling

We investigate the qubit geometric phase and its properties in dependence on the mechanism for decoherence of a qubit weakly coupled to its environment. We consider two sources of decoherence: dephasing coupling (without exchange of energy with environment) and dissipative coupling (with exchange of energy). Reduced dynamics of the qubit is studied in terms of the rigorous Davies Markovian quantum master equation, both at zero and non--zero temperature. For pure dephasing coupling, the geometric phase varies monotonically with respect to the polar angle (in the Bloch sphere representation) parameterizing an initial state of the qubit. Moreover, it is antisymmetric about some points on the geometric phase-polar angle plane. This is in distinct contrast to the case of dissipative coupling for which the variation of the geometric phase with respect to the polar angle typically is non-monotonic, displaying local extrema and is not antisymmetric. Sensitivity of the geometric phase to details of the decoherence source can make it a tool for testing the nature of the qubit--environment interaction.Comment: accepted for publication in Quantum Information Processin

Absolute negative mobility induced by white Poissonian noise

We research the transport properties of inertial Brownian particles which move in a symmetric periodic potential and are subjected to both a symmetric, unbiased time-periodic external force and biased Poissonian white shot noise (of non-zero average F) being composed of a random sequence of delta-shaped pulses with random amplitudes. Upon varying the parameters of white shot-noise one conveniently can manipulate the transport direction and the overall nonlinear response behavior. Within tailored parameter regimes, we find that the response is opposite to the applied average bias F of such white shot noise. This very transport characteristics thus mimics a nonlinear Absolute Negative Mobility (ANM) regime. Moreover, such white shot noise driven ANM is robust with respect to statistics of the shot noise spikes. Our findings can be checked and corroborated experimentally by use of a setup that consists of a single resistively and capacitively shunted Josephson junction device.Comment: 14 pages, 12 figures; accepted in J. Stat. Mech.: Theor. Exp. (2013

Absolute negative mobility induced by thermal equilibrium fluctuations

A novel transport phenomenon is identified that is induced by inertial Brownian particles which move in simple one-dimensional, symmetric periodic potentials under the influence of both a time periodic and a constant, biasing driving force. Within tailored parameter regimes, thermal equilibrium fluctuations induce the phenomenon of absolute negative mobility (ANM), which means that the particle noisily moves {\it backwards} against a small constant bias. When no thermal fluctuations act, the transport vanishes identically in these tailored regimes. There also exist parameter regimes, where ANM can occur in absence of fluctuations on grounds which are rooted solely in the complex, inertial deterministic dynamics. The experimental verification of this new transport scheme is elucidated for the archetype symmetric physical system: a convenient setup consisting of a resistively and capacitively shunted Josephson junction device.Comment: 4 pages, 3 figures. Phys. Rev. Lett. (in press

Comment on "White-Noise-Induced Transport in Periodic Structures"

In the paper by J.\L uczka {\em et al.} ({\em Europhys. Lett.}, {\bf 31} (1995) 431), the authors reported by rigorous calculation that an additive Poissonian white shot noise can induce a macroscopic current of a dissipative particle in a periodic potential -- even {\em in the absence} of spatial asymmetry of the potential. We argue that their main result is an obvious one caused by the spatially broken symmetry of a probability distribution of the additive noise, unlike the similar result caused by chaotic noise which has a symmetric probability distribution ({\em J.Phys.Soc.Jpn.}, {\bf 63} (1994) 2014).Comment: 2 pages (Latex); submitted to Europhys.Let

Negative conductances of Josephson junctions: Voltage fluctuations and energetics

We study a resistively and capacitively shunted Josephson junction, which is driven by a combination of time-periodic and constant currents. Our investigations concern three main problems: (A) The voltage fluctuations across the junction; (B) The quality of transport expressed in terms of the P\'eclet number; (C) The efficiency of energy transduction from external currents. These issues are discussed in different parameter regimes that lead to: (i) absolute negative conductance; (ii) negative differential conductance, and (iii) normal, Ohmic-like conductance. Conditions for optimal operation of the system are studied.Comment: 7 pages, 4 figures, Presented at the "Frontiers of Quantum and Mesoscopic Thermodynamics", 28 July - 2 August 2008, Prague, Czech Republi

Frequency Windows of Absolute Negative Conductance in Josephson Junctions

We report on anomalous conductance in a resistively and capacitively shunted Josephson junction which is simultaneously driven by ac and dc currents. The dependence of the voltage across the junction on the frequency of the ac current shows windows of absolute negative conductance regimes, i.e. for a positive (negative) dc current, the voltage is negative (positive).Comment: 4 pages, 1 figur

Dynamics at the angle of repose: jamming, bistability, and collapse

When a sandpile relaxes under vibration, it is known that its measured angle of repose is bistable in a range of values bounded by a material-dependent maximal angle of stability; thus, at the same angle of repose, a sandpile can be stationary or avalanching, depending on its history. In the nearly jammed slow dynamical regime, sandpile collapse to a zero angle of repose can also occur, as a rare event. We claim here that fluctuations of {\it dilatancy} (or local density) are the key ingredient that can explain such varied phenomena. In this work, we model the dynamics of the angle of repose and of the density fluctuations, in the presence of external noise, by means of coupled stochastic equations. Among other things, we are able to describe sandpile collapse in terms of an activated process, where an effective temperature (related to the density as well as to the external vibration intensity) competes against the configurational barriers created by the density fluctuations.Comment: 15 pages, 1 figure. Minor changes and update