36 research outputs found
Entanglement dynamics for two harmonic oscillators coupled to independent environments
We study the entanglement evolution between two harmonic oscillators having
different free frequencies each leaking into an independent bath. We use an
exact solution valid in the weak coupling limit and in the short time
non-Markovian regime. The reservoirs are identical and characterized by an
Ohmic spectral distribution with Lorents-Drude cut-off. This work is an
extension of the case reported in [Phys. Rev. A 80, 062324 (2009)] where the
oscillators have the same free frequency.Comment: 8 pages, 3 figures, submitted to Physica Script
Time resolved quantum dynamics of double ionization in strong laser fields
Quantum calculations of a 1+1-dimensional model for double ionization in strong laser fields are used to trace the time evolution from the ground state through ionization and rescattering to the two electron escape. The subspace of symmetric escape, a prime characteristic of nonsequential double ionization, remains accessible by a judicious choice of 1-d coordinates for the electrons. The time resolved ionization fluxes show the onset of single and double ionization, the sequence of events during the pulse, and the influences of pulse duration, and reveal the relative importance of sequential and non-sequential double ionization, even when ionization takes place during the same field cycle
Quantum model for double ionization of atoms in strong laser fields
We discuss double ionization of atoms in strong laser pulses using a reduced
dimensionality model. Following the insights obtained from an analysis of the
classical mechanics of the process, we confine each electron to move along the
lines that point towards the two-particle Stark saddle in the presence of a
field. The resulting effective two dimensional model is similar to the aligned
electron model, but it enables correlated escape of electrons with equal
momenta, as observed experimentally. The time-dependent solution of the
Schr\"odinger equation allows us to discuss in detail the time dynamics of the
ionization process, the formation of electronic wave packets and the
development of the momentum distribution of the outgoing electrons. In
particular, we are able to identify the rescattering process, simultaneous
direct double ionization during the same field cycle, as well as other double
ionization processes. We also use the model to study the phase dependence of
the ionization process.Comment: 14 pages, 16 figures, version accepted for publication in Phys. Rev.
Dynamics of quantum entanglement in Gaussian open systems
In the framework of the theory of open systems based on completely positive
quantum dynamical semigroups, we give a description of the dynamics of
entanglement for a system consisting of two uncoupled harmonic oscillators
interacting with a thermal environment. Using Peres-Simon necessary and
sufficient criterion for separability of two-mode Gaussian states, we describe
the evolution of entanglement in terms of the covariance matrix for a Gaussian
input state. For some values of the temperature of environment, the state keeps
for all times its initial type: separable or entangled. In other cases,
entanglement generation, entanglement sudden death or a repeated collapse and
revival of entanglement take place. We determine the asymptotic Gaussian
maximally entangled mixed states (GMEMS) and their corresponding asymptotic
maximal logarithmic negativity.Comment: 10 pages, 2 figures; talk at the International Workshop on Quantum
Non-Stationary Systems, Brasilia, Brazil (2009
Entanglement dynamics of bipartite system in squeezed vacuum reservoirs
Entanglement plays a crucial role in quantum information protocols, thus the
dynamical behavior of entangled states is of a great importance. In this paper
we suggest a useful scheme that permits a direct measure of entanglement in a
two-qubit cavity system. It is realized in the cavity-QED technology utilizing
atoms as fying qubits. To quantify entanglement we use the concurrence. We
derive the conditions, which assure that the state remains entangled in spite
of the interaction with the reservoir. The phenomenon of sudden death
entanglement (ESD) in a bipartite system subjected to squeezed vacuum reservoir
is examined. We show that the sudden death time of the entangled states depends
on the initial preparation of the entangled state and the parameters of the
squeezed vacuum reservoir.Comment: 10 pages, 5 figures, CEWQO17(St Andrews
Multi-mode entanglement of N harmonic oscillators coupled to a non-Markovian reservoir
Multi-mode entanglement is investigated in the system composed of coupled
identical harmonic oscillators interacting with a common environment. We treat
the problem very general by working with the Hamiltonian without the
rotating-wave approximation and by considering the environment as a
non-Markovian reservoir to the oscillators. We invoke an -mode unitary
transformation of the position and momentum operators and find that in the
transformed basis the system is represented by a set of independent harmonic
oscillators with only one of them coupled to the environment. Working in the
Wigner representation of the density operator, we find that the covariance
matrix has a block diagonal form that it can be expressed in terms of multiples
of and matrices. This simple property allows to treat
the problem to some extend analytically. We illustrate the advantage of working
in the transformed basis on a simple example of three harmonic oscillators and
find that the entanglement can persists for long times due to presence of
constants of motion for the covariance matrix elements. We find that, in
contrast to what one could expect, a strong damping of the oscillators leads to
a better stationary entanglement than in the case of a weak damping.Comment: 21 pages, 4 figure
Characterization of bipartite states using a single homodyne detector
We suggest a scheme to reconstruct the covariance matrix of a two-mode state
using a single homodyne detector plus a polarizing beam splitter and a
polarization rotator. It can be used to fully characterize bipartite Gaussian
states and to extract relevant informations on generic states.Comment: 7 pages, 1 figur
Markovian Master Equations: A Critical Study
We derive Markovian master equations of single and interacting harmonic
systems in different scenarios, including strong internal coupling. By
comparing the dynamics resulting from the corresponding Markovian master
equations with exact numerical simulations of the evolution of the global
system, we precisely delimit their validity regimes and assess the robustness
of the assumptions usually made in the process of deriving the reduced
dynamics. The proposed method is sufficiently general to suggest that the
conclusions made here are widely applicable to a large class of settings
involving interacting chains subject to a weak interaction with an environment.Comment: 40 pages, 14 figures, final versio
Quantifying decoherence in continuous variable systems
We present a detailed report on the decoherence of quantum states of
continuous variable systems under the action of a quantum optical master
equation resulting from the interaction with general Gaussian uncorrelated
environments. The rate of decoherence is quantified by relating it to the decay
rates of various, complementary measures of the quantum nature of a state, such
as the purity, some nonclassicality indicators in phase space and, for two-mode
states, entanglement measures and total correlations between the modes.
Different sets of physically relevant initial configurations are considered,
including one- and two-mode Gaussian states, number states, and coherent
superpositions. Our analysis shows that, generally, the use of initially
squeezed configurations does not help to preserve the coherence of Gaussian
states, whereas it can be effective in protecting coherent superpositions of
both number states and Gaussian wave packets.Comment: Review article; 36 pages, 19 figures; typos corrected, references
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