Search results for "master"

Transitionless quantum driving in open quantum systems

2014

Abstract We extend the concept of superadiabatic dynamics, or transitionless quantum driving, to quantum open systems whose evolution is governed by a master equation in the Lindblad form. We provide the general framework needed to determine the control strategy required to achieve superadiabaticity. We apply our formalism to two examples consisting of a two-level system coupled to environments with time-dependent bath operators.

Limits in the characteristic function description of non-Lindblad-type open quantum systems

2005

In this paper I investigate the usability of the characteristic functions for the description of the dynamics of open quantum systems focussing on non-Lindblad-type master equations. I consider, as an example, a non-Markovian generalized master equation containing a memory kernel which may lead to nonphysical time evolutions characterized by negative values of the density matrix diagonal elements [S.M. Barnett and S. Stenholm, Phys. Rev. A {\bf 64}, 033808 (2001)]. The main result of the paper is to demonstrate that there exist situations in which the symmetrically ordered characteristic function is perfectly well defined while the corresponding density matrix loses positivity. Therefore no…

Dynamical Ising-like model for the two-step spin-crossover systems

2003

In order to reproduce the two-step relaxation observed experimentally in spin-crossover systems, we investigate analytically the static and the dynamic properties of a two-sublattice Ising-like Hamiltonian. The formalism is based on a stochastic master equation approach. It is solved in the mean-field approximation, and yields two coupled differential equations that correspond to the HS fractions of the sublattices A and B. Virginie.Niel@uv.es ; Jose.A.Real@uv.es

Non-Markovian Wave Function Simulations of Quantum Brownian Motion

2005

The non-Markovian wave function method (NMWF) using the stochastic unravelling of the master equation in the doubled Hilbert space is implemented for quantum Brownian motion. A comparison between the simulation and the analytical results shows that the method can be conveniently used to study the non-Markovian dynamics of the system.

Dissipation and decoherence in Brownian motion

2007

We consider the evolution of a Brownian particle described by a measurement-based master equation. We derive the solution to this equation for general initial conditions and apply it to a Gaussian initial state. We analyse the effects of the diffusive terms, present in the master equation, and describe how these modify uncertainties and coherence length. This allows us to model dissipation and decoherence in quantum Brownian motion.

The relaxation-time limit in the quantum hydrodynamic equations for semiconductors

2006

Abstract The relaxation-time limit from the quantum hydrodynamic model to the quantum drift–diffusion equations in R 3 is shown for solutions which are small perturbations of the steady state. The quantum hydrodynamic equations consist of the isentropic Euler equations for the particle density and current density including the quantum Bohm potential and a momentum relaxation term. The momentum equation is highly nonlinear and contains a dispersive term with third-order derivatives. The equations are self-consistently coupled to the Poisson equation for the electrostatic potential. The relaxation-time limit is performed both in the stationary and the transient model. The main assumptions are…

Dissipative effects on a generation scheme of a W state in an array of coupled Josephson junctions

2011

The dynamics of an open quantum system, consisting of three superconducting qubits interacting with independent reservoirs, is investigated to elucidate the effects of the environment on a unitary generation scheme of W states (Migliore R et al 2006 Phys. Rev. B 74 104503). To this end a microscopic master equation is constructed and its exact resolution predicts the generation of a Werner-like state instead of the W state. A comparison between our model and a more intuitive phenomenological model is also considered, in order to find the limits of the latter approach in the case of structured reservoirs.

2015

The recently proposed Wigner function for a particle in an infinite lattice [NJP 14, 103009 (2012)] is extended here to include an internal degree of freedom, as spin. The formalism is developed to account for dynamical processes, with or without decoherence. We show explicit solutions for the case of Hamiltonian evolution under a position-dependent potential, and for evolution governed by a master equation under some simple models of decoherence. Discrete processes are also discussed. Finally we discuss the possibility of introducing a negativity concept for the Wigner function in the case in which the spin degree of freedom is included.

A Coherent Master Equation for active mode locking in lasers

2015

We present the derivation of a new master equation for active mode locking in lasers that fully takes into account the coherent effects of the light matter interaction through a peculiar adiabatic elimination technique. The coherent effects included in our model could be relevant to describe properly mode-locked semiconductor lasers where the standard Haus’ Master Equation predictions show some discrepancy with respect to the experimental results and can be included in the modelling of other mode locking techniques too.

Probabilistic description of traffic flow

2005

Abstract A stochastic description of traffic flow, called probabilistic traffic flow theory, is developed. The general master equation is applied to relatively simple models to describe the formation and dissolution of traffic congestions. Our approach is mainly based on spatially homogeneous systems like periodically closed circular rings without on- and off-ramps. We consider a stochastic one-step process of growth or shrinkage of a car cluster (jam). As generalization we discuss the coexistence of several car clusters of different sizes. The basic problem is to find a physically motivated ansatz for the transition rates of the attachment and detachment of individual cars to a car cluster…