Measurements, quantum discord and parity in spin 1 systems

We consider the evaluation of the quantum discord and other related measures of quantum correlations in a system formed by a spin 1 and a complementary spin system. A characterization of general projective measurements in such system in terms of spin averages is thereby introduced, which allows to easily visualize their deviation from standard spin measurements. It is shown that the measurement optimizing these measures corresponds in general to a non-spin measurement. The important case of states that commute with the total $S_z$ spin parity is discussed in detail, and the general stationary measurements for such states (parity preserving measurements) are identified. Numerical and analytical results for the quantum discord, the geometric discord and the one way information deficit in the relevant case of a mixture of two aligned spin 1 states are also presented.Comment: 6 pages, 2 figures, References adde

Generalized mean field description of entanglement in dimerized spin systems

We discuss a generalized self-consistent mean field (MF) treatment, based on the selection of an arbitrary subset of operators for representing the system density matrix, and its application to the problem of entanglement evaluation in composite quantum systems. As a specific example, we examine in detail a pair MF approach to the ground state (GS) of dimerized spin 1/2 systems with anisotropic ferromagnetic-type XY and XYZ couplings in a transverse field, including chains and arrays with first neighbor and also longer range couplings. The approach is fully analytic and able to capture the main features of the GS of these systems, in contrast with the conventional single spin MF. Its phase diagram differs significantly from that of the latter, exhibiting (Sz) parity breaking just in a finite field window if the coupling between pairs is sufficiently weak, together with a fully dimerized phase below this window and a partially aligned phase above it. It is then shown that through symmetry restoration, the approach is able to correctly predict not only the concurrence of a pair, but also its entanglement with the rest of the chain, which shows a pronounced peak in the parity breaking window. Perturbative corrections allow to reproduce more subtle observables like the entanglement between weakly coupled spins and the low lying energy spectrum. All predictions are tested against exact results for finite systems.Comment: 13 pages, 9 figures. Final versio

Coherent control of quantum systems as a resource theory

Control at the interface between the classical and the quantum world is fundamental in quantum physics. In particular, how classical control is enhanced by coherence effects is an important question both from a theoretical as well as from a technological point of view. In this work, we establish a resource theory describing this setting and explore relations to the theory of coherence, entanglement and information processing. Specifically, for the coherent control of quantum systems the relevant resources of entanglement and coherence are found to be equivalent and closely related to a measure of discord. The results are then applied to the DQC1 protocol and the precision of the final measurement is expressed in terms of the available resources.Comment: 9 pages, 4 figures, final version. Discussions were improved and some points were clarified. The title was slightly changed to agree with the published versio

Factorization and entanglement in general XYZ spin arrays in non-uniform transverse fields

We determine the conditions for the existence of a pair of degenerate parity breaking separable eigenstates in general arrays of arbitrary spins connected through $XYZ$ couplings of arbitrary range and placed in a transverse field, not necessarily uniform. Sufficient conditions under which they are ground states are also provided. It is then shown that in finite chains, the associated definite parity states, which represent the actual ground state in the immediate vicinity of separability, can exhibit entanglement between any two spins regardless of the coupling range or separation, with the reduced state of any two subsystems equivalent to that of pair of qubits in an entangled mixed state. The corresponding concurrences and negativities are exactly determined. The same properties persist in the mixture of both definite parity states. These effects become specially relevant in systems close to the $XXZ$ limit. The possibility of field induced alternating separable solutions with controllable entanglement side limits is also discussed. Illustrative numerical results for the negativity between the first and the $j^{\rm th}$ spin in an open spin $s$ chain for different values of $s$ and $j$ are as well provided.Comment: 6 pages, figures adde

A multilayer panel in cork and natural phase change materials: thermal and energy analysis

This paper presents thermal and energy analysis of a multilayer panel in bio-based cork material and natural phase change materials (PCMs) for the development of prefabricated, recyclable and energy-efficient and autonomous building modules. For this purpose, a calculation tool is developed for the dynamic simulation of the thermal and energy behaviour of the sandwich panel. In particular, through an extensive parametric survey, the panel is sized with the identification of the arrangement of the layers, PCM temperature, and layer thicknesses to optimize the insulating and damping properties, considering typical climatic conditions of the Mediterranean climates of Southern Italy. From the conducted simulations, the types of sandwich panels that have the best insulating and storage characteristics for the building module construction were chosen. The results of these simulations will be used in future research for the preliminary design of tests to be carried out in a climatic chamber and to build a building module in real conditions to be constantly monitored through the automatic instrumental survey of internal and external physical quantities such as temperature, humidity and radiant temperature

Thermal entanglement in fully connected spin systems and its RPA description

We examine the thermal pairwise entanglement in a symmetric system of $n$ spins fully connected through anisotropic $XYZ$-type couplings embedded in a transverse magnetic field. We consider both the exact evaluation together with that obtained with the static path + random phase approximation (RPA) and the ensuing mean field + RPA. The latter is shown to provide an accurate analytic description of both the parallel and antiparallel thermal concurrence in large systems. We also analyze the limit temperature for pairwise entanglement, which is shown to increase for large fields and to decrease logarithmically with increasing $n$. Special finite size effects are as well discussed.Comment: 9 pages, 5 figure

Description of thermal entanglement with the static path plus random-phase approximation

We discuss the application of the static path plus random phase approximation (SPA+RPA) and the ensuing mean field+RPA treatment to the evaluation of entanglement in composite quantum systems at finite temperature. These methods involve just local diagonalizations and the determination of the generalized collective vibrational frequencies. As illustration, we evaluate the pairwise entanglement in a fully connected XXZ chain of $n$ spins at finite temperature in a transverse magnetic field $b$. It is shown that already the mean field+RPA provides an accurate analytic description of the concurrence below the mean field critical region ($|b|<b_c$), exact for large $n$, whereas the full SPA+RPA is able to improve results for finite systems in the critical region. It is proved as well that for $T>0$ weak entanglement also arises when the ground state is separable ($|b|>b_c$), with the limit temperature for pairwise entanglement exhibiting quite distinct regimes for $|b|b_c$.Comment: 20 pages, 5 figure

Path Integrals from Spacetime Quantum Actions

We present a spacetime Hilbert space formulation of Feynman path integrals (PIs). It relies on a tensor product structure in time which provides extended representations of dynamical quantum observables through a spacetime quantum action operator. As a consequence, the ``sum over paths'' of the different PI formulations naturally arise within the same Hilbert space, with each one associated with a different quantum trajectory basis. New insights on PI-based results naturally follow, including exact discretizations and a non-trivial approach to the continuum limit.Comment: 8 pages, 1 figur