arXiv subjects

Marcelo Franca Santos

Publications and source records attributed to Marcelo Franca Santos.

9 recordsLinked to original sources

arXiv2010

Physically Realizable Entanglement by Local Continuous Measurements

Quantum systems prepared in pure states evolve into mixtures under environmental action. Physically realizable ensembles are the pure state decompositions of those mixtures that can be generated in time through continuous measurements of the environment. Here, we define physically realizable entanglement as the average entanglement over realizable ensembles. We optimize the measurement strategy to maximize and minimize this quantity through local observations on the independent environments that cause two qubits to disentangle in time. We then compare it with the entanglement bounds for the unmonitored system. For some relevant noise sources the maximum realizable entanglement coincides with the upper bound, establishing the scheme as an alternative to locally protect entanglement. However, for local strategies, the lower bound of the unmonitored system is not reached.

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arXiv2007

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.

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arXiv2007

A proposal for the implementation of quantum gates with photonic-crystal coupled cavity waveguides

Quantum computers require technologies that offer both sufficient control over coherent quantum phenomena and minimal spurious interactions with the environment. We show, that photons confined to photonic crystals, and in particular to highly efficient waveguides formed from linear chains of defects doped with atoms can generate strong non-linear interactions which allow to implement both single and two qubit quantum gates. The simplicity of the gate switching mechanism, the experimental feasibility of fabricating two dimensional photonic crystal structures and integrability of this device with optoelectronics offers new interesting possibilities for optical quantum information processing networks.

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arXiv2006

Entanglement versus energy in the entanglement transfer problem

We study the relation between energy and entanglement in an entanglement transfer problem. We first analyze the general setup of two entangled qubits (a and b) exchanging this entanglement with two other independent qubits (A and B). Qubit a (b) interacts with qubit A (B) via a spin exchange-like unitary evolution. A physical realization of this scenario could be the problem of two-level atoms transferring entanglement to resonant cavities via independent Jaynes-Cummings interactions. We study the dynamics of entanglement and energy for the second pair of qubits (tracing out the originally entangled ones) and show that these quantities are closely related. For example, the allowed quantum states occupy a restricted area in a phase diagram entanglement vs. energy. Moreover the curve which bounds this area is exactly the one followed if both interactions are equal and the entire four qubit system is isolated. We also consider the case when the target pair of qubits is subjected to losses and can spontaneously decay.

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arXiv2006

Geometric phase induced by a cyclically evolving squeezed vacuum reservoir

We propose a new way to generate an observable geometric phase by means of a completely incoherent phenomenon. We show how to imprint a geometric phase to a system by "adiabatically" manipulating the environment with which it interacts. As a specific scheme we analyse a multilevel atom interacting with a broad-band squeezed vacuum bosonic bath. As the squeezing parameters are smoothly changed in time along a closed loop, the ground state of the system acquires a geometric phase. We propose also a scheme to measure such geometric phase by means of a suitable polarization detection.

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arXiv2006

Coherent evolution via reservoir driven holonomy

We show that in the limit of strongly interacting environment a system initially prepared in a Decoherence Free Subspace (DFS) coherently evolves in time, adiabatically following the changes of the DFS. If the reservoir cyclicly evolves in time, the DFS states acquire an holonomy.

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arXiv2004

Universal and deterministic manipulation of the quantum state of harmonic oscillators: a route to unitary gates for Fock State qubits

We present a simple quantum circuit that allows for the universal and deterministic manipulation of the quantum state of confined harmonic oscillators. The scheme is based on the selective interactions of the referred oscillator with an auxiliary three-level system and a classical external driving source, and enables any unitary operations on Fock states, two-by-two. One circuit is equivalent to a single qubit unitary logical gate on Fock states qubits. Sequences of similar protocols allow for complete, deterministic and state-independent manipulation of the harmonic oscillator quantum state.

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arXiv2004

Anyons and transmutation of statistics via vacuum induced Berry phase

We show that bosonic fields may present anyonic behavior when interacting with a fermion in a Jaynes-Cummings-like model. The proposal is accomplished via the interaction of a two-level system with two quantized modes of a harmonic oscillator; under suitable conditions, the system acquires a fractional geometric phase. A crucial role is played by the entanglement of the system eigenstates, which provides a two-dimensional confinement in the effective evolution of the system, leading to the anyonic behavior. For a particular choice of parameters, we show that it is possible to transmute the statistics of the system continually from fermions to bosons. We also present an experimental proposal, in an ion-trap setup, in which fractional statistical features can be generated, controlled, and measured.

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arXiv2004

Accessibility of physical states and non-uniqueness of entanglement measure

Ordering physical states is the key to quantifying some physical property of the states uniquely. Bipartite pure entangled states are totally ordered under local operations and classical communication (LOCC) in the asymptotic limit and uniquely quantified by the well-known entropy of entanglement. However, we show that mixed entangled states are partially ordered under LOCC even in the asymptotic limit. Therefore, non-uniqueness of entanglement measure is understood on the basis of an operational notion of asymptotic convertibility.

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