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 $n$-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

Factorized ground state in dimerized spin chains

PACS numbers: 03.67.Mn, 75.10.Jm, 64.70.TgThe possibility of observing factorized ground states in dimerized spin systems is studied. A set of sufficient conditions is derived which allows one to establish whether or not it is possible to have factorization both in nearest-neighbour and long-range Hamiltonians. These conditions can be derived by forcing factorization for each of the pairwise terms of the total Hamiltonian. Due to the peculiar structure of a dimerized chain, an antiferromagnetic factorized ground state of the kind | %i, | %i, | -i, | -i (forbidden in regular chains) is possible.This work was funded by the ECusCo project no. 2008501047. The author is supported by the Spanish Ministry of Science and Innovation through the programme Juan de la CiervaPeer reviewe