997 research outputs found
Multipartite quantum and classical correlations in symmetric n-qubit mixed states
We discuss how to calculate genuine multipartite quantum and classical
correlations in symmetric, spatially invariant, mixed -qubit density
matrices. We show that the existence of symmetries greatly reduces the amount
of free parameters to be optimized in order to find the optimal measurement
that minimizes the conditional entropy in the discord calculation. We apply
this approach to the states exhibited dynamically during a thermodynamic
protocol to extract maximum work. We also apply the symmetry criterion to a
wide class of physically relevant cases of spatially homogeneous noise over
multipartite entangled states. Exploiting symmetries we are able to calculate
the nonlocal and genuine quantum features of these states and note some
interesting properties.Comment: Close to published Versio
Quantum state transfer in the presence of non homogeneous external potentials
Heisenberg-type spin models in the limit of a low number of excitations are
useful tools to study basic mechanisms in strongly correlated and magnetic
systems. Many of these mechanisms can be experimentally tested using ultracold
atoms. Here, we discuss the implementation of a quantum state transfer protocol
in a tight-binding chain in the presence of an inhomogeneous external
potential. We show that it can be used to extend the parameter range in which
high fidelity state transfer can be achieved beyond the well established
weak-coupling regime. Among the class of mirror-reflecting potentials that
allow for high-fidelity quantum state transfer, the harmonic case is especially
relevant because it allows us to formulate a proposal for the experimental
implementation of the protocol in the context of optical lattices
Correlation approach to work extraction from finite quantum systems
Reversible work extraction from identical quantum systems via collective
operations was shown to be possible even without producing entanglement among
the sub-parts. Here, we show that implementing such global operations
necessarily imply the creation of quantum correlations, as measured by quantum
discord. We also reanalyze the conditions under which global transformations
outperform local gates as far as maximal work extraction is considered by
deriving a necessary and sufficient condition that is based on classical
correlations
Genuine correlations in finite-size spin systems
Genuine multipartite correlations in finite-size XY chains are studied as a
function of the applied external magnetic field. We find that, for low
temperatures, multipartite correlations are sensitive to the parity change in
the Hamiltonian ground state, given that they exhibit a minimum every time that
the ground state becomes degenerate. This implies that they can be used to
detect the factorizing point, that is, the value of the external field such
that, in the termodynamical limit, the ground state becomes the tensor product
of single-spin states.Comment: Submitted to Int. J. Mod. Phys. B, special issue "Classical Vs
Quantum correlations in composite systems" edited by L. Amico, S. Bose, V.
Korepin and V. Vedra
Two-spin entanglement induced by electron scattering in nanostructures
We present a model where two magnetic impurities in a discrete tight-binding
ring become entangled because of scattering processes associated to the
injection of a conduction electron. We introduce a weak coupling approximation
that allows us to solve the problem in a analytical way and compare the theory
with the exact numerical results. We obtain the generation of entanglement both
in a deterministic way and in a probabilistic one. The first case is
intrinsically related to the structure of the two-impurity reduced density
matrix, while the second one occurs when a projection on the electron state is
performed
Probing the spectral density of a dissipative qubit via quantum synchronization
The interaction of a quantum system, which is not accessible by direct
measurement, with an external probe can be exploited to infer specific features
of the system itself. We introduce a probing scheme based on the emergence of
spontaneous quantum synchronization between an out-of-equilibrium qubit, in
contact with an external environment, and a probe qubit. Tuning the frequency
of the probe leads to a transition between synchronization in phase and
antiphase. The sharp transition between these two regimes is locally accessible
by monitoring the probe dynamics alone and allows one to reconstruct the shape
of the spectral density of the environment
Quantum Darwinism and non-Markovian dissipative dynamics from quantum phases of the spin-1/2 XX model
Quantum Darwinism explains the emergence of a classical description of
objects in terms of the creation of many redundant registers in an environment
containing their classical information. This amplification phenomenon, where
only classical information reaches the macroscopic observer and through which
different observers can agree on the objective existence of such object, has
been revived lately for several types of situations, successfully explaining
classicality. We explore quantum Darwinism in the setting of an environment
made of two level systems which are initially prepared in the ground state of
the XX model, which exhibits different phases; we find that the different
phases have different ability to redundantly acquire classical information
about the system, being the "ferromagnetic phase" the only one able to complete
quantum Darwinism. At the same time we relate this ability to how non-Markovian
the system dynamics is, based on the interpretation that non-Markovian dynamics
is associated to back flow of information from environment to system, thus
spoiling the information transfer needed for Darwinism. Finally, we explore
mixing of bath registers by allowing a small interaction among them, finding
that this spoils the stored information as previously found in the literature
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