M. O. Terra Cunha

Quasiprobability and Probability Distributions for Spin-1/2 States

We develop a Radon-like transformation, in which P quasiprobability distribution for spin-1/2 states is written in terms of the tomographic probability distribution w.

Asymptotic entanglement dynamics and geometry of quantum states

A given dynamics for a composite quantum system can exhibit several distinct properties for the asymptotic entanglement behavior, such as entanglement sudden death, asymptotic death of entanglement, sudden birth of entanglement, etc. A classification of the possible situations was given by Terra Cunha (2007 New J. Phys. 9 237) but for some classes there were no known examples. In this work, we give a better classification for the possible relaxing dynamics in light of the geometry of their set of asymptotic states and give explicit examples for all the classes. Although the classification is completely general, in the search for examples it is sufficient to use two qubits with dynamics given by differential equations in the Lindblad form (some of them are non-autonomous). We also investigate, in each case, the probabilities of finding each possible behavior for random initial states.

Are all maximally entangled states pure

We study if all maximally entangled states are pure through several entanglement monotones. In the bipartite case, we find that the same conditions which lead to the uniqueness of the entropy of entanglement as a measure of entanglement exclude the existence of maximally mixed entangled states. In the multipartite scenario, our conclusions allow us to generalize the idea of the monogamy of entanglement: we establish the polygamy of entanglement, expressing that if a general state is maximally entangled with respect to some kind of multipartite entanglement, then it is necessarily factorized of any other system.

Minimal state tomography of spatial qubits using a spatial light modulator

We report minimal quantum state tomography with spatial qubits created by a pair of parametric down converted twin-photons passing through a double-slit. A novel experimental setup is used, which includes a Spatial Light Modulator, as a fundamental tool, to reconstruct the state density matrix. The theory needed to perform a minimal quantum tomography is described. The density matrix is experimentally obtained for the two-qubit photonic states in spatial variables.

Useful entanglement from the Pauli principle

We address the question whether identical-particle entanglement is a useful resource for quantum information processing. We answer this question positively by reporting a scheme to create entanglement using semiconductor quantum wells. The Pauli exclusion principle forces quantum correlations between the spins of two independent fermions in the conduction band. Selective electron-hole recombination then transfers this entanglement to the polarization of emitted photons, which can subsequently be used for quantum information tasks.

Realistic loophole-free Bell test with atom-photon entanglement.

The establishment of nonlocal correlations, guaranteed through the violation of a Bell inequality, is not only important from a fundamental point of view but constitutes the basis for device-independent quantum information technologies. Although several nonlocality tests have been conducted so far, all of them suffered from either locality or detection loopholes. Among the proposals for overcoming these problems are the use of atom-photon entanglement and hybrid photonic measurements (for example, photodetection and homodyning). Recent studies have suggested that the use of atom-photon entanglement can lead to Bell inequality violations with moderate transmission and detection efficiencies. Here we combine these ideas and propose an experimental setup realizing a simple atom-photon entangled state that can be used to obtain nonlocality when considering realistic experimental parameters including detection efficiencies and losses due to required propagation distances.

Multipartite entanglement of superpositions

The entanglement of superpositions [Linden et al., Phys. Rev. Lett. 97, 100502 (2006)]is generalized to the multipartite scenario: an upper bound to the multipartite entanglement of a superposition is given in terms of the entanglement of the superposed states and the superposition coefficients. This bound is proven to be tight for a class of states composed of an arbitrary number of qubits. We also extend the result to a large family of quantifiers, which includes the negativity, the robustness of entanglement, and the best separable approximation measure.

Protection of quantum information and optimal singlet conversion through higher-dimensional quantum systems and environment monitoring

We study how to protect quantum information in quantum systems subjected to local dissipation. We show that combining the use of three-level systems, environment monitoring, and local feedback can fully and deterministically protect any available quantum information, including entanglement initially shared by different parties. These results can represent a gain in resources and/or distances in quantum communication protocols such as quantum repeaters and teleportation as well as time for quantum memories. Finally, we show that monitoring local environments physically implements the optimum singlet conversion protocol, which is essential for classical entanglement percolation.

A Thermodynamic Formalism for Density Matrices in Quantum Information

We consider new concepts of entropy and pressure for stationary systems acting on density matrices which generalize the usual ones in Ergodic Theory. Part of our work is to justify why the denitions and results we describe here are natural generalizations of the classical concepts of Thermo- dynamic Formalism (in the sense of R. Bowen, Y. Sinai and D. Ruelle). It is well-known that the concept of density operator should replace the concept of measure for the cases in which we consider a quantum formalism. We consider the operatoracting on the space of density matr ices MN over anite N-dimensional complex Hilbert space �( �) := k X i=1 tr(WiW � i ) ViV � i tr(ViV � i ) ,

Decay rate and decoherence control in coupled dissipative cavities

We give a detailed account of the derivation of a master equation for two-coupled cavities in the presence of dissipation. The analytical solution is presented and physical limits of interest are discussed. Firstly, we show that the decay rate of initial coherent states can be significantly modified if the two cavities have different decay rates and are weakly coupled through a wire. Moreover, we show that also decoherence rates can be substantially altered by manipulation of physical parameters. Conditions for experimental realizations are discussed.