Sabrina Maniscalco

Quantifying non-Markovianity of continuous-variable Gaussian dynamical maps

We introduce a non-Markovianity measure for continuous-variable open quantum systems based on the idea put forward in H.-P. Breuer et al.[Phys. Rev. Lett. 103, 210401 (2009);], that is, by quantifying the flow of information from the environment back to the open system. Instead of the trace distance we use here the fidelity to assess distinguishability of quantum states. We employ our measure to evaluate non-Markovianity of two paradigmatic Gaussian channels: the purely damping channel and the quantum Brownian motion channel with Ohmic environment. We consider different classes of Gaussian states and look for pairs of states maximizing the backflow of information. For coherent states we find simple analytical solutions, whereas for squeezed states we provide both exact numerical and approximate analytical solutions in the weak coupling limit.

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

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 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, nonphysical situations may not show up in the characteristic function. As a consequence, the characteristic function cannot be considered an alternative complete description of the non-Lindblad dynamics.

Interaction of bimodal fields with few-level atoms in cavities and traps

Abstract The spectacular experimental results of the last few years in cavity quantum electrodynamics and trapped-ion research has led to very-high-level laboratory performances. Such a stimulating situation essentially stems from two decisive advancements. The first is the invention of reliable protocols for the manipulation of single atoms. The second is the ability to produce desired bosonic environments on demand. This progress has led to the possibility of controlling the form of the coupling between individual atoms and an arbitrary number of bosonic modes. As a consequence, fundamental matter-radiation interaction models, for instance, the Jaynes-Cummings model and most of its numerous nonlinear multiphoton generalizations, have been realized or simulated in the laboratory and their dynamic features have been tested more or less in detail. This topical paper reviews the state of the art of the theoretical investigations and of the experimental observations concerning the dynamic features of the coupling between single few-level atoms and two bosonic modes. In the course of the paper we show that such a configuration provides an excellent platform for investigating various quantum intermode correlation effects tested or testable in the cavity quantum electrodynamics and trapped ion experimental realms. In particular we discuss a mode-mode correlation effect appearing in the dynamics of a two-level atom quadratically coupled to two bosonic modes. This effect, named the parity effect, consists of a high sensitivity to the evenness or oddness of the total number of bosonic excitations.

Misbeliefs and misunderstandings about the non-Markovian dynamics of a damped harmonic oscillator

We use the exact solution for the damped harmonic oscillator to discuss some relevant aspects of its open dynamics are often misunderstood. We compare two different approximations both referred to as a rotating wave approximation. Using a specific example, we clarify some issues related to non-Markovian dynamics, non-Lindblad-type dynamics, and positivity of the density matrix.

Entanglement control via reservoir engineering in ultracold atomic gases

We study the entanglement of two impurity qubits immersed in a Bose-Einstein condensate (BEC) reservoir. This open quantum system model allows for interpolation between a common dephasing scenario and an independent dephasing scenario by modifying the wavelength of the superlattice superposed to the BEC, and how this influences the dynamical properties of the impurities. We demonstrate the existence of rich dynamics corresponding to different values of reservoir parameters, including phenomena such as entanglement trapping, revivals of entanglement, and entanglement generation. In the spirit of reservoir engineering, we present the optimal BEC parameters for entanglement generation and trapping, showing the key role of the ultracold-gas interactions.

Quantum theory of heating of a single trapped ion

Abstract The heating of trapped ions due to the interaction with a quantized environment is studied without performing the Born–Markov approximation . A generalized master equation local in time is derived and a novel theoretical approach to solve it analytically is proposed. Our master equation is in the Lindblad form with time dependent coefficients, thus allowing the simulation of the dynamics by means of the Monte Carlo Wave Function (MCWF) method.

Comparison between the rotating wave and Feynman-Vernon system-reservoir couplings in the non-Markovian regime

This paper deals with the dissipative dynamics of a quantum harmonic oscillator interacting with a bosonic reservoir. The Master Equations based on the Rotating Wave and on the Feynman-Vernon system--reservoir couplings are compared highlighting differences and analogies. We discuss quantitatively and qualitatively the conditions under which the counter rotating terms can be neglected. By comparing the analytic solution of the heating function relative to the two different coupling models we conclude that, even in the weak coupling limit, the counter rotating terms give rise to a significant contribution in the non--Markovian short time regime. The main result of this paper is that such a contribution is actually experimentally measurable and thus relevant for a correct description of the system dynamics.

Non-Markovian qubit dynamics induced by Coulomb crystals

We investigate the backflow of information in a system with a second-order structural phase transition, namely, a quasi-one-dimensional Coulomb crystal. Using standard Ramsey interferometry which couples a target ion (the system) to the rest of the chain (a phononic environment), we study the non-Markovian character of the resulting open system dynamics. We study two different time scales and show that the backflow of information pinpoints both the phase transition and different dynamical features of the chain as it approaches criticality. We also establish an exact link between the backflow of information and the Ramsey fringe visibility.

Generation of entanglement density within a reservoir

We study a single two-level atom interacting with a reservoir of modes defined by its reservoir structure function. Within this framework we are able to define a density of entanglement involving a continuum of reservoir modes. The density of entanglement is derived for a system with a single excitation by taking a limit of the global entanglement. Utilizing the density of entanglement we quantify the entanglement between the atom and the modes, and also between the reservoir modes themselves.

Preparation of macroscopically distinguishable superpositions of circular or linear oscillatory states of a bidimensionally trapped ion

Abstract A simple scheme for the generation of two different classes of bidimensional vibrational Schrödinger cat-like states of an isotropically trapped ion is presented. We show that by appropriately adjusting an easily controllable parameter having a clear physical meaning, the states prepared by our procedure are quantum superpositions of either vibrational axial angular momentum eigenstates or Fock states along two orthogonal directions.