Method for measuring two-qubit entanglement of formation by local operations and classical communication

We present a parametrically efficient method for measuring the entanglement of formation E-f in an arbitrarily given unknown two-qubit state rho(AB) by local operations and classical communication. The two observers, Alice and Bob, first perform some local operations on their composite systems separately, by which the desired global quantum states can be prepared. Then they estimate seven functions via two modified local quantum networks supplemented a classical communication. After obtaining these functions, Alice and Bob can determine the concurrence C and the entanglement of formation E-f.

Many-body entanglement in decoherence processes

A pure state decoheres into a mixed state as it entangles with an environment. When an entangled two-mode system is embedded in a thermal environment, however, each mode may not be entangled with its environment by their simple linear interaction. We consider an exactly solvable model to study the dynamics of a total system, which is composed of an entangled two-mode system and a thermal environment. The Markovian interaction with the environment is concerned with an array of infinite number of beam splitters. It is shown that many-body entanglement of the system and the environment may play a crucial role in the process of disentangling the system.

Quantum Discord Bounds the Amount of Distributed Entanglement

The ability to distribute quantum entanglement is a prerequisite for many fundamental tests of quantum theory and numerous quantum information protocols. Two distant parties can increase the amount of entanglement between them by means of quantum communication encoded in a carrier that is sent from one party to the other. Intriguingly, entanglement can be increased even when the exchanged carrier is not entangled with the parties. However, in light of the defining property of entanglement stating that it cannot increase under classical communication, the carrier must be quantum. Here we show that, in general, the increase of relative entropy of entanglement between two remote parties is bounded by the amount of nonclassical correlations of the carrier with the parties as quantified by the relative entropy of discord. We study implications of this bound, provide new examples of entanglement distribution via unentangled states, and put further limits on this phenomenon.

Distillable entanglement and area laws in spin and harmonic-oscillator systems

We address the presence of nondistillable (bound) entanglement in natural many-body systems. In particular, we consider standard harmonic and spin-1/2 chains, at thermal equilibrium and characterized by few interaction parameters. The existence of bound entanglement is addressed by calculating explicitly the negativity of entanglement for different partitions. This allows us to individuate a range of temperatures for which no entanglement can be distilled by means of local operations, despite the system being globally entangled. We discuss how the appearance of bound entanglement can be linked to entanglement-area laws, typical of these systems. Various types of interactions are explored, showing that the presence of bound entanglement is an intrinsic feature of these systems. In the harmonic case, we analytically prove that thermal bound entanglement persists for systems composed by an arbitrary number of particles. Our results strongly suggest the existence of bound entangled states in the macroscopic limit also for spin-1/2 systems.

Thermal bound entanglement in macroscopic systems and area law

Does bound entanglement naturally appear in quantum many-body systems? We address this question by showing the existence of bound-entangled thermal states for harmonic oscillator systems consisting of an arbitrary number of particles. By explicit calculations of the negativity for different partitions, we find a range of temperatures for which no entanglement can be distilled by means of local operations, despite the system being globally entangled. We offer an interpretation of this result in terms of entanglement-area laws, typical of these systems. Finally, we discuss generalizations of this result to other systems, including spin chains.

Miscibility in a degenerate fermionic mixture induced by linear coupling

We consider a one-dimensional mean-field-hydrodynamic model of a two-component degenerate Fermi gas in an external trap, each component representing a spin state of the same atom. We demonstrate that the interconversion between them (linear coupling), imposed by a resonant electromagnetic wave, transforms the immiscible binary gas into a miscible state, if the coupling constant, kappa, exceeds a critical value, kappa(cr). The effect is predicted in a variational approximation, and confirmed by numerical solutions. Unlike the recently studied model of a binary Bose-Einsten condensate with the linear coupling, the components in the immiscible phase of the binary fermion mixture never fill two separated domains with a wall between them, but rather form antilocked (pi-phase-shifted) density waves. Another difference from the bosonic mixture is spontaneous breaking of symmetry between the two components in terms of the numbers of atoms in them, N(1) and N(2). The latter effect is characterized by the parameter nu equivalent to(N(1)-N(2))/(N(1)+N(2)) (only N(1)+N(2) is a conserved quantity), the onset of miscibility at kappa >=kappa(cr) meaning a transition to nu equivalent to 0. At kappa

Mixing, demixing, and structure formation in a binary dipolar Bose-Einstein condensate

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Vortex lattice and matching fields for a long superconducting wire

We investigate the flux penetration patterns and matching fields of a long cylindrical wire of circular cross section in the presence of an external magnetic field. For this study we write the London theory for a long cylinder both for the mixed and Meissner states, with boundary conditions appropriate for this geometry. Using the Monte Carlo simulated annealing method, the free energy of the mixed state is minimized with respect to the vortex position and we obtain the ground state of the vortex lattice for N=3 up to 18 vortices. The free energy of the Meissner and mixed states provides expressions for the matching fields. We find that, as in the case of samples of different geometry, the finite-size effect provokes a delay on the vortex penetration and a vortex accumulation in the center of the sample. The vortex patterns obtained are in good agreement with experimental results.

Size effects in the magnetization of a superconducting wire

The size effects in the magnetization of a long cylindrical wire of circular cross section in the presence of an external magnetic field are investigated. For this study the London theory is used with boundary conditions appropriate for this geometry. Using the Monte Carlo simulated annealing method, the free energy of the mixed state is minimized with respect to the vortex positions. The ground state of the vortex lattice for n = 1 up to 18 vortices for a given radius of the cylinder is obtained. It is found that the finite size of the sample provokes a matching effect in the magnetization, as found in experiments with superconducting samples of finite size but different geometry. © 1999 American Institute of Physics.

Matching fields of a long superconducting film

We obtain the vortex configurations, the matching fields, and the magnetization of a superconducting film with a finite cross section. The applied magnetic field is normal to this cross section, and we use the London theory to calculate many of its properties, such as the local magnetic field, the free energy, and the induction for the mixed state. Thus previous similar theoretical works, done for an infinitely long superconducting film, are recovered here, in the special limit of a very long cross section. ©1999 The American Physical Society.

Mixing-demixing in a trapped degenerate fermion-fermion mixture

We use a time-dependent dynamical mean-field-hydrodynamic model to study mixing-demixing in a degenerate fermion-fermion mixture (DFFM). It is demonstrated that with the increase of interspecies repulsion and/or trapping frequencies, a mixed state of a DFFM could turn into a fully demixed state in both three-dimensional spherically symmetric as well as quasi-one-dimensional configurations. Such a demixed state of a DFFM could be experimentally realized by varying an external magnetic field near a fermion-fermion Feshbach resonance, which will result in an increase of interspecies fermion-fermion repulsion, and/or by increasing the external trap frequencies. © 2006 The American Physical Society.

Magnetic field profile of a mesoscopic SQUID-shaped superconducting film

Using a genuinely tridimensional approach to the time-dependent Ginzburg-Landau theory, we have studied the local magnetic field profile of a mesoscopic superconductor in the so-called SQUID geometry, i.e., a square with a hole at the center connected to the outside vacuum through a very thin slit. Our investigation was carried out in both the Meissner and the mixed state. We have also studied the influence of the temperature on the space distribution of the local magnetic field. © 2013 IOP Publishing Ltd.

COMPLEXITY MEASURE: A QUANTUM INFORMATION APPROACH

In the past decades, all of the efforts at quantifying systems complexity with a general tool has usually relied on using Shannon's classical information framework to address the disorder of the system through the Boltzmann-Gibbs-Shannon entropy, or one of its extensions. However, in recent years, there were some attempts to tackle the quantification of algorithmic complexities in quantum systems based on the Kolmogorov algorithmic complexity, obtaining some discrepant results against the classical approach. Therefore, an approach to the complexity measure is proposed here, using the quantum information formalism, taking advantage of the generality of the classical-based complexities, and being capable of expressing these systems' complexity on other framework than its algorithmic counterparts. To do so, the Shiner-Davison-Landsberg (SDL) complexity framework is considered jointly with linear entropy for the density operators representing the analyzed systems formalism along with the tangle for the entanglement measure. The proposed measure is then applied in a family of maximally entangled mixed state.

Präparation, Charakterisierung und elektronische Eigenschaften von dünnen Schichten des Hochtemperatursupraleiters HgReBa 2 Ca n-1 Cu n O y

Ziel dieser Arbeit war die Pr"{a}paration, Charakterisierung und Untersuchung der elektronischen Eigenschaften von d"{u}nnen Schichten des Hochtemperatursupraleiters HgReBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{y}$, die mittels gepulster Laser-Deposition hergestellt wurden. Die HgRe1212-Filme zeigen in der AC-Suszeptibilit"{a}t einen scharfen "{U}bergang in die supraleitende Phase bei 124 K mit einer "{U}bergangsbreite von 2 K. Die resistiven "{U}berg"{a}nge der Proben wurden mit zunehmender St"{a}rke des externen Magnetfeldes breiter. Aus der Steigung der Arrheniusplots konnte die Aktivierungsenergie f"{u}r verschiedene Feldst"{a}rken bestimmt werden. Weiterhin wurde die Winkelabh"{a}ngigkeit des Depinning-Feldes $B_{dp}(theta)$ der Filme gemessen. Hieraus wurde ein Anisotropiewert von $gamma$ = 7.7 bei 105 K ermittelt. Dies ist relevant, um den f"{u}r Anwendungen wichtigen Bereich im $T$-$B$-$theta$-Phasenraum des Materials absch"{a}tzen zu k"{o}nnen. Die kritische Stromdichte $J_{c}$ der d"{u}nnen Filme aus HgRe-1212 wurde mit Hilfe eines SQUID-Magnetometers gemessen. Die entsprechenden $M$-$H$ Kurven bzw. das magnetische Moment dieser Filme wurde f"{u}r einen weiten Temperatur- und Feldbereich mit einem magnetischen Feld senkrecht zum Film aufgenommen. F"{u}r einen HgRe-1212-Film konnte bei 5 K eine kritische Stromdichte von 1.2 x 10$^{7}$ A/cm$^{2}$ und etwa 2 x 10$^{6}$ A/cm$^{2}$ bei 77 K ermittelt werden. Es wurde die Magnetfeld- und die Temperaturabh"{a}ngigkeit des Hall-Effekts im normalleitenden und im Mischzustand in Magnetfeldern senkrecht zur $ab$-Ebene bis zu 12 T gemessen. Oberhalb der kritischen Temperatur $T_{c}$ steigt der longitudinale spezifische Widerstand $rho_{xx}$ linear mit der Temperatur, w"{a}hrend der spezifische Hall-Widerstand $rho_{yx}$ sich umgekehrt proportional zur Temperatur "{a}ndert. In der N"{a}he von $T_{c}$ und in Feldern kleiner als 3 T wurde eine doppelte Vorzeichen"{a}nderung des spezifischen Hall-Widerstandes beobachtet. Der Hall-Winkel im Normalzustand, cot $theta_{H}= alpha T^{2} + beta$, folgt einer universellen $textit{T }^{2}$-Abh"{a}ngigkeit in allen magnetischen Feldern. In der N"{a}he des Nullwiderstand-Zustandes h"{a}ngt der spezifische Hall-Widerstand $rho_{yx}$ "{u}ber ein Potenzgesetz mit dem longitudinalen Widerstand $rho_{xx}$ zusammen. Das Skalenverhalten zwischen $rho_{yx}$ und $rho_{xx}$ weist eine starke Feld-Abh"{a}ngigkeit auf. Der Skalenexponent $beta$ in der Gleichung $rho_{yx}$ =A $rho_{xx}^{beta}$ steigt von 1.0 bis 1.7, w"{a}hrend das Feld von 1.0 bis 12 T zunimmt.

Quantum hydrodynamic models for semiconductors with and without collisions

In dieser Arbeit werden Quantum-Hydrodynamische (QHD) Modelle betrachtet, die ihren Einsatz besonders in der Modellierung von Halbleiterbauteilen finden. Das QHD Modell besteht aus den Erhaltungsgleichungen für die Teilchendichte, das Momentum und die Energiedichte, inklusive der Quanten-Korrekturen durch das Bohmsche Potential. Zu Beginn wird eine Übersicht über die bekannten Ergebnisse der QHD Modelle unter Vernachlässigung von Kollisionseffekten gegeben, die aus ein­em Schrödinger-System für den gemischten-Zustand oder aus der Wigner-Glei­chung hergeleitet werden können. Nach der Reformulierung der eindimensionalen QHD Gleichungen mit linearem Potential als stationäre Schrö­din­ger-Gleichung werden die semianalytischen Fassungen der QHD Gleichungen für die Gleichspannungs-Kurve betrachtet. Weiterhin werden die viskosen Stabilisierungen des QHD Modells be­rück­sich­tigt, sowie die von Gardner vorgeschlagene numerische Viskosität für das {sf upwind} Finite-Differenzen Schema berechnet. Im Weiteren wird das viskose QHD Modell aus der Wigner-Glei­chung mit Fokker-Planck Kollisions-Ope­ra­tor hergeleitet. Dieses Modell enthält die physikalische Viskosität, die durch den Kollision-Operator eingeführt wird. Die Existenz der Lösungen (mit strikt positiver Teilchendichte) für das isotherme, stationäre, eindimensionale, viskose Modell für allgemeine Daten und nichthomogene Randbedingungen wird gezeigt. Die dafür notwendigen Abschätzungen hängen von der Viskosität ab und erlauben daher den Grenzübergang zum nicht-viskosen Fall nicht. Numerische Simulationen der Resonanz-Tunneldiode modelliert mit dem nichtisothermen, stationären, eindimensionalen, viskosen QHD Modell zeigen den Einfluss der Viskosität auf die Lösung. Unter Verwendung des von Degond und Ringhofer entwickelten Quanten-Entropie-Minimierungs-Verfahren werden die allgemeinen QHD-Gleichungen aus der Wigner-Boltzmann-Gleichung mit dem BGK-Kollisions-Operator hergeleitet. Die Herleitung basiert auf der vorsichtige Entwicklung des Quanten-Max­well­ians in Potenzen der skalierten Plankschen Konstante. Das so erhaltene Modell enthält auch vertex-Terme und dispersive Terme für die Ge­schwin­dig­keit. Dadurch bleibt die Gleichspannungs-Kurve für die Re­so­nanz-Tunnel­diode unter Verwendung des allgemeinen QHD Modells in einer Dimension numerisch erhalten. Die Ergebnisse zeigen, dass der dispersive Ge­schwin­dig­keits-Term die Lösung des Systems stabilisiert.