5 research outputs found
Unruh effect as a result of quantization of spacetime
A way to encode acceleration directly into fields has recently being
proposed, thus establishing a new kind of fields, the accelerated fields. The
definition of accelerated fields points to the quantization of space and time,
analogously to the way quantities like energy and momentum are quantized in
usual quantum field theories. Unruh effect has been studied in connection with
quantum field theory in curved spacetime and it is described by recruiting a
uniformly accelerated observer. In this work, as a first attempt to demonstrate
the utility of accelerated fields, we present an alternative way to derive
Unruh effect. We show, by studying quantum field theory on quantum spacetime,
that Unruh effect can be obtained without changing the reference frame. Thus,
in the framework of accelerated fields, the observational confirmation of Unruh
effect could be assigned to the existence of quantum properties of spacetime.Comment: Comments are welcom
Decoherence of a composite particle induced by a weak quantized gravitational field
Despite the fact that we have some proposals for the quantum theory of
gravity like string theory or loop quantum gravity, we do not have any
experimental evidence supporting any of these theories. Actually, we do not
have experimental evidence pointing in the direction that we really need a
quantum description of the gravitational field. In this scenario, several
proposals for experimentally investigating quantum gravitational effects far
from Plank scale have recently appear in literature, like gravitationally
induced entanglement, for instance. An important issue of theses approaches is
the decoherence introduced by the quantum nature not only of the system under
consideration, but also from the gravitational field itself. Here, by means of
the Feynman-Vernon influence functional we study the decoherence of a quantum
system induced by the quantized gravitational field and by its own quantum
nature. Our results may be important in providing a better understanding of
many phenomena like the decoherence induced by the gravitational time-dilation,
the quantum reference frames and the quantum equivalence principle.Comment: Comments are welcom
Reassessing thermodynamic advantage from indefinite causal order
Indefinite causal order is a key feature involved in the study of quantum
higher order transformations. Recently, intense research has been focused on
possible advantages related to the lack of definite causal order of quantum
processes. Quite often the quantum switch is claimed to provide advantages in
information-theoretic and thermodynamic tasks. We address here the question
whether indefinite causal order is a resource for quantum thermodynamics.
Inspired by previous results in the literature, we show that indefinite causal
order is not necessary for the reported increase in free energy and ergotropy.
More specifically, we show that a simple causally ordered process, which
replaces the system's state with a new one before the final measurement,
outperforms the quantum switch in all thermodynamic tasks considered so far. We
further show that a similar advantage can be also achieved without completely
discarding system, if we allow for non-Markovian interactions between the
system and an environment. We extend the analysis to more extreme examples of
indefinite causal order, showing that they do not provide an advantage either.
Finally, we discuss a possible way to study the advantages that may arise from
indefinite causal order in a general scenario
O efeito Casimir dinâmico e decoerência
In this thesis we first demonstrate that the inevitable action of the environment can be substantially weakened when considering appropriate nonstationary quantum systems. Beyond protecting quantum states against decoherence, an oscillating frequency can be engineered to make the system-reservoir coupling almost negligible. Differently from the program for engineering reservoir and similarly to the schemes for dynamical decoupling of open quantum systems, our technique does not require a previous knowledge of the state to be protected. However, differently from the previously-reported schemes for dynamical decoupling, our technique does not rely on the availability of tailored external pulses acting faster than the shortest time scale accessible to the reservoir degree of freedom. We show, in the domain of cavity quantum electrodynamics, how to engineer such a nonstationary cavity mode through its dispersive interaction with a driven two-level atom. Next, we consider different aspects of the dynamical Casimir effect (DCE) through the derivation of effective Hamiltonians which exhibit the essential features of the phenomenon. We start by investigating the dynamical Casimir effect in a nonideal cavity at finite temperature. We first compute a general expression for the average number of particle creation, applicable for any law of motion of the cavity boundary. We also compute a general expression for the linear entropy of an arbitrary state prepared in a selected mode, also applicable for any law of motion of the cavity boundary. As an application of our results we have analyzed both the average number of particle creation and linear entropy within a particular oscillatory motion of the cavity boundary. On the basis of these expressions we develop a comprehensive analysis of the resonances in the number of particle creation in the nonideal dynamical Casimir effect. We also demonstrate the occurrence of resonances in the loss of purity of the initial state and estimate the decoherence times associated with these resonances. We also consider the dynamical Casimir effect for a massless scalar field, under Dirichlet boundary conditions, between two concentric spherical shells. We obtain a general expression for vii the average number of particle creation, for an arbitrary law of radial motion of the spherical shells, using two distinct methods: by computing the density operator of the system and by calculating the Bogoliubov coefficients. We apply our general expression to breathing modes: when only one of the shells oscillates and when both shells oscillate in or out of phase. We also analyze the number of particle production and compare it with the results for the case of plane geometry. Finally, we analyze the action of the gravitational field on the dynamical Casimir effect. We consider a massless scalar field confined in a cuboid cavity placed in a gravitational field described by a static and diagonal metric. With one of the plane mirrors of the cavity allowed to move, we compute the average number of particles created inside the cavity by means of the Bogoliubov coefficients computed through perturbative expansions. We apply our result to the case of an oscillatory motion of the mirror, considering a weak gravitational field described by the Schwarzschild metric. The regime of parametric amplification is detailed analyzed, demonstrating that our computed result, for the mean number of particles created, is in agreement with associated particular cases in literature.Universidade Federal de Sao CarlosDemonstramos nesta tese, primeiramente, que a ação inevitável do meio ambiente pode ser substancialmente enfraquecida quando consideramos sistemas quânticos nĂŁo estacionários apropriados. Diferentemente do programa de engenharia de reservatĂłrios e de forma similar aos protocolos para o desacoplamento dinâmico de sistemas quânticos abertos, nossa proposta nĂŁo requer o conhecimento prĂ©vio do estado a ser protegido. Diferentemente mesmo dos esquemas de desacoplamento dinâmico, a proteção de estados de modos nĂŁo estacionários prescinde da disponibilidade de pulsos externos ultra rápidos que atuem sobre o sistema de interesse em intervalos de tempo mais curtos que a menor escala de tempo acessĂvel aos graus de liberdade do reservatĂłrio. No domĂnio da engenharia de estados em eletrodinâmica quântica de cavidades, mostramos como preparar modos nĂŁo estacionários atravĂ©s da interação dispersiva do campo com átomos de dois nĂveis submetidos `a amplificação linear. Tratamos, em seguida, de diferentes aspectos do efeito Casimir dinâmico (ECD) atravĂ©s da derivação de hamiltonianos efetivos capazes de descrever convenientemente os principais aspectos do fen omeno. Começamos pelo ECD nĂŁo ideal a temperaturas finitas. Obtivemos expressões gerais tanto para nĂşmero mĂ©dio de partĂculas criadas como para a entropia linear de um estado arbitrário preparado em um modo selecionado da cavidade, expressões estas que se aplicam a qualquer lei de movimento da fronteira mĂłvel. Desenvolvemos, atravĂ©s destas expressões, uma análise abrangente das ressonâncias presentes tanto na criação de partĂculas como na perda de pureza e decoerĂŞncia de estados. Consideramos tambĂ©m o ECD no contexto de um campo escalar nĂŁo massivo confinado, sob as condições de contorno de Dirichlet, entre duas cascas esfĂ©ricas concĂŞntricas. Utilizamos dois diferentes mĂ©todos, o operador densidade e os coeficientes de Bogoliubov, para o cálculo da expressĂŁo geral para o nĂşmero mĂ©dio de partĂculas criadas, válida para qualquer lei de movimento das cascas esfĂ©ricas. Aplicamos esta expressĂŁo para o cálculo do nĂşmero de partĂculas produzidas quando apenas uma das cascas oscila ou quando ambas oscilam em fase ou fora de fase, e v comparamos os resultados com aqueles associados `a geometria plana. Por fim, analisamos a ação do campo gravitacional sobre o ECD. Para tal, consideramos um campo escalar nĂŁo massivo confinado numa cavidade cubĂłide localizada em um campo gravitacional descrito por uma mĂ©trica estática e diagonal. Assumindo que uma das paredes planas da cavidade seja mĂłvel, obtivemos o nĂşmero de partĂculas criadas no interior da cavidade atravĂ©s do cálculo, via expansões perturbativas, dos coeficientes de Bogoliubov. Aplicamos o resultado para o caso particular de um movimento oscilatĂłrio da fronteira mĂłvel, considerando um campo gravitacional fraco descrito pela mĂ©trica de Schwarzschild. O regime de amplificação paramĂ©trica Ă© detalhadamente analisado, demonstrando que nosso resultado para o nĂşmero mĂ©dio de partĂculas criadas, está em acordo com resultados particulares previamente apresentados na literatura