Orientador: Marcos César de Oliveira / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin. / Made available in DSpace on 2018-08-13T03:07:21Z (GMT). No. of bitstreams: 1 Steinhoff_FrankEduardodaSilva_M.pdf: 633661 bytes, checksum: 4c5305fba85a1d80713ed5dfe1f90f32 (MD5) Previous issue date: 2009 / Resumo: Propomos um método para obtenção de propriedades de sistemas quânticos compostos utilizando apenas medições estritamente locais e comunicação clássica. Isso difere dos esquemas usualmente utilizados em protocolos de informação quântica, que se utilizam em sua maioria de operações globais e/ou operações locais conjuntas. Nosso tratamento consiste em analisar o efeito de medições em um subsistema sobre as submatrizes da matriz densidade do sistema composto. Analisamos então como outros subsistemas podem ser expressos em termos dessas submatrizes, obtendo assim o efeito das medições feitas em um subsistema sobre os outros. Aplicamos esse resultado - válido para sistemas com espaços de Hilbert discretos de dimensão arbitrária - a dois problemas centrais em protocolos de informação quântica: a reconstrução de estados de sistemas compostos e a caracterização de emaranhamento. Para a tarefa de reconstrução, mostramos como determinar um estado de um sistema composto de dimensão finita arbitrária utilizando apenas medições locais e comunicação clássica de uma via. A vantagem de nossa proposta está em eliminar a necessidade de medições conjuntas, o que se traduz, no contexto de ótica linear, em eliminar medições de coincidência. Analisamos a caracterização de emaranhamento considerando classes especiais de estados. Para estados com alta simetria - estados isotrópicos, de Werner e de simetria rotacional constituídos por um qubit e um qudit - mostramos uma relação entre o grau de emaranhamento e a diferença de população medida por um subsistema condicionada a medições de paridade do outro subsistema, relação essa já evidenciada em estados gaussianos simétricos. Além disso, propomos uma nova família de estados cujo emaranhamento pode ser caracterizado com muito menos recursos do que os utilizados para a reconstrução, sendo esses recursos estritamente locais. / Abstract: We propose a method to obtain properties of composite quantum systems using strictly local measurements and classical communication only. This differs from schemes usually employed in quantum information protocols where global and/or joint local operations are commonly used. Our treatment consists of analysing the effect of measurements of a system over submatrices of the density matrix of the compound system. We analyse then how copies of a subsystem can be expressed in terms of these submatrices, obtaining thus the effect of the measurements done in a subsystem upon the others. We apply this result - valid for discrete Hilbert spaces of arbitrary dimension - to two central problems in quantum information protocols: the state reconstruction of composite systems and entanglement characterization. For the reconstruction task, we show how to determine a state of a arbitrary finite dimension composite system using local measurements and one-way classical communnication only. The advantage of our proposal lies in elimnating the need for joint measurements, which translates as eliminating coincidence measurements in linear optics context. We analyse entanglement characterization considering special classes of states. For high symmetry states -isotropic states, Werner states and rotationally symmetric states composed of a qubit and a qudit - we show a relation between entanglement degree and population difference measured by one subsystem conditioned to parity measurements of the other subsystem, this relation already present in gaussian symmetric states. Moreover, we propose a new family os states whose entanglement can be characterized with much fewer resources than that used for reconstruction, this resources being strictly local. / Mestrado / Física / Mestre em Física

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We investigate two different coupling schemes between a nano-scale mechanical resonator and one-electron atoms. In these schemes, classical electromagnetic radiation mediates a mutual communication between the mechanical resonator and the atoms. In the process it generates atomic coherences, quantum superpositions of excited electronic levels of the atoms. An atomic coherence is highly responsive to subtle variations in the relative frequencies of the levels participating in such superposition state. By exposing the atoms to electromagnetic radiation modulated by the motion of the mechanical resonator, we show how the response of an atomic coherence can, under appropriate conditions, be used to affect on demand the dynamical state of the mechanical resonator. The first scheme realizes a long range interface between a mechanical resonator and an ensemble of three-level atoms. Here, mechanically modulated electromagnetic radiation comes from a laser beam reflected off an oscillating mirror, the mechanical resonator. This light beam drives the transition between an excited level and a hyperfine sublevel of the atoms with a certain detuning. A weaker light beam resonantly couples to the transition between the excited level and another hyperfine sublevel. On full resonance, the atoms evolve into a stationary coherence of the above (non-absorbing) hyperfine sublevels only. The atoms then become transparent to the weaker light beam, in a phenomenon called electromagnetically induced transparency. Off resonance, we find that this transparency is modulated at the mirror frequency with some phase shift, which allows the weaker beam to cause resonant backaction onto the moving mirror. The strength of this backaction is enhanced near atomic resonances and its character can be switched between amplification or damping of mirror vibrations by adjusting the detuning. In contrast, the second scheme accomplishes a closer range interface between a torsion pendulum and guided two level Rydberg atoms. Attaching a point electric dipole to the torsion pendulum allows electromagnetic coupling to two Rydberg levels of a passing atom. This coupling modifies the eigenfrequencies of the Rydberg levels such that they become dependent on the phonon number of the torsion pendulum. Via Ramsey interferometry, we may readout this effect and thus measure the phonon number. We show that, by subjecting several atoms, one by one, to a Ramsey measurement, a quantum non-demolition detection of the phonon number is feasible. Likewise, we show coherent oscillator displacements possible, by driving the atoms with external fields while they interact with the torsion pendulum. We propose a protocol to reconstruct the quantum state of motion of the torsion pendulum, combining these two techniques, Ramsey measurements and oscillator displacements. Our interfaces between a mechanical resonator and atoms provide alternative routes for the control of the state of motion, ultimately quantum mechanical, of a mechanical resonator, in which the latter is not restricted to be part of a cavity. We will thus ease quantum dynamical manipulations of mechanical resonators of sub micron scales, for which an efficient design of cavity opto- and electro-mechanical systems is hard.