Entanglement universality of two-qubit X-states

We demonstrate that for every two-qubit state there is a X-counterpart, i.e., a corresponding two-qubit X-state of same spectrum and entanglement, as measured by concurrence, negativity or relative entropy of entanglement. By parametrizing the set of two-qubit X-states and a family of unitary transformations that preserve the sparse structure of a two-qubit X-state density matrix, we obtain the parametric form of a unitary transformation that converts arbitrary two-qubit states into their X-counterparts. Moreover, we provide a semi-analytic prescription on how to set the parameters of this unitary transformation in order to preserve concurrence or negativity. We also explicitly construct a set of X-state density matrices, parametrized by their purity and concurrence, whose elements are in one-to-one correspondence with the points of the concurrence versus purity (CP) diagram for generic two-qubit states.Comment: 24 pages, 6 figures. v2 includes new references and minor changes (accepted version

Generalized squeezing operators, bipartite Wigner functions and entanglement via Wehrl's entropy functionals

We introduce a new class of unitary transformations based on the su(1,1) Lie algebra that generalizes, for certain particular representations of its generators, well-known squeezing transformations in quantum optics. To illustrate our results, we focus on the two-mode bosonic representation and show how the parametric amplifier model can be modified in order to generate such a generalized squeezing operator. Furthermore, we obtain a general expression for the bipartite Wigner function which allows us to identify two distinct sources of entanglement, here labelled by dynamical and kinematical entanglement. We also establish a quantitative estimate of entanglement for bipartite systems through some basic definitions of entropy functionals in continuous phase-space representations.Comment: 16 page

Quasiprobability distribution functions for finite-dimensional discrete phase spaces: Spin-tunneling effects in a toy model

We show how quasiprobability distribution functions defined over $N^{2}$-dimensional discrete phase spaces can be used to treat physical systems described by a finite space of states which exhibit spin tunneling effects. This particular approach is then applied to the Lipkin-Meshkov-Glick model in order to obtain the time evolution of the discrete Husimi function, and as a by-product the energy gap for a symmetric combination of ground and first excited states. Moreover, we also show how an angle-based potential approach can be efficiently employed to explain qualitatively certain features of the energy gap in terms of a spin tunneling. Entropy functionals are also discussed in this context. Such results reinforce not only the formalism per se but also the possibility of some future potential applications in other branches of physics.Comment: 7 pages, 8 figures, title modified, new setences and references included, to appear in Physical Review

Stretching and unpairing in the \"yrast\" line

Estiramento e desemparelhamento na linha de \"yrast\"Stretching and unpairing in the \"yrast\" lin

Isoscalar monopolar vibrations in some doubly magic nuclei.

Usamos o formalismo de A. Toledo Pisa e E. Passos, que usa o método de coordenadas geradores e o Teorema Espectral de Anélise Funcional, para obter uma hamiltoniana coletiva para as vibrações monopolares isoescalares nos núcleos duplamente mágicos ANPOT. 4 He, ANPOT. 16 O e ANPOT 40 C. As funções de onda do oscilador hermônico são tomadas como funções de onda geradoras, e a interação de muitos corpos é aquela de Skyrme. Com esta hamiltoniana coletiva calculamos o espectro de energias do modo monopolar isoescalar, parâmetros de inércia e módulos de incompressibilidade para aqueles núcleos e comparamos com os resultados de Flocard e Vautherin, que resolvem numericamente a equação de Griffin-Wheeler. Foram calculados ainda os raios quadráticos médios nucleares. Os efeitos espúrios dos movimentos do centro de massa são eliminados exatamente para o AMPOT. 4 He e recalculamos aquelas propriedades nucleares.The isoscalar monopole vibrational mode in 4 He,16 O and 40Ca is studied with the aid of collective Hamiltonians obtained from e general formalism proposed by A. Toledo Piza and E. Passos, which makes use of the generator coordinate method and Functional Analisis Spectral Theorem. Cur generator wave functions are that of the harmonic oscillator and use is made of the Skyrme\'s schematic effective interaction. Using the collective Hamiltonian we can calculate the energy spectrum, inertial parameters end incompressibility modulus and compare with numerical results of Flocard and Vautherin. R. m. s. radii are also calculated. Center of mass spurious effects are exactly eliminated and the forementioned nuclear properties are calculated again for 4 He

Isoscalar monopolar vibrations in some doubly magic nuclei.

Usamos o formalismo de A. Toledo Pisa e E. Passos, que usa o método de coordenadas geradores e o Teorema Espectral de Anélise Funcional, para obter uma hamiltoniana coletiva para as vibrações monopolares isoescalares nos núcleos duplamente mágicos ANPOT. 4 He, ANPOT. 16 O e ANPOT 40 C. As funções de onda do oscilador hermônico são tomadas como funções de onda geradoras, e a interação de muitos corpos é aquela de Skyrme. Com esta hamiltoniana coletiva calculamos o espectro de energias do modo monopolar isoescalar, parâmetros de inércia e módulos de incompressibilidade para aqueles núcleos e comparamos com os resultados de Flocard e Vautherin, que resolvem numericamente a equação de Griffin-Wheeler. Foram calculados ainda os raios quadráticos médios nucleares. Os efeitos espúrios dos movimentos do centro de massa são eliminados exatamente para o AMPOT. 4 He e recalculamos aquelas propriedades nucleares.The isoscalar monopole vibrational mode in 4 He,16 O and 40Ca is studied with the aid of collective Hamiltonians obtained from e general formalism proposed by A. Toledo Piza and E. Passos, which makes use of the generator coordinate method and Functional Analisis Spectral Theorem. Cur generator wave functions are that of the harmonic oscillator and use is made of the Skyrme\'s schematic effective interaction. Using the collective Hamiltonian we can calculate the energy spectrum, inertial parameters end incompressibility modulus and compare with numerical results of Flocard and Vautherin. R. m. s. radii are also calculated. Center of mass spurious effects are exactly eliminated and the forementioned nuclear properties are calculated again for 4 He