Termodinâmica e classicalidade

Orientador: Marcos César de OliveiraDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb WataghinResumo: Máquinas térmicas que operam no regime quântico são dispositivos compostos por um ou mais sistemas quânticos que operam ciclicamente entre dois reservatórios cujo objetivo é produzir trabalho. Embora as leis da mecânica quântica governem esses dispositivos, isso não implica que essas máquinas funcionem de maneira quântica ou que exibam alguma vantagem. De fato, máquinas quânticas mostraram uma notável semelhança com os modelos clássicos, levantando assim à questão de \emph{o que é quântico em termodinâmica quântica}. Com a ajuda do conceito de P-representabilidade, apresentamos uma simples expressão, denominada função de classicalidade, que permite quantificar o grau de não-classicalidade de um sistema bosônico. Como ilustração, consideramos uma máquina quântica que consiste em um único modo bosônico, como substância de trabalho, acoplada a um reservatório térmico frio e um reservatório térmico comprimido quente. Para dois protocolos específicos, ou seja, um ciclo de Otto quântico e um ciclo generalizado, mostramos que a não-classicalidade é um recurso para executar tarefas termodinâmicas mais eficientes. Além disso, o presente estudo apresenta um arcabouço teórico para estudar como recursos quânticos, como emaranhamento ou coerência, afetam as formulações clássicas dos axiomas da termodinâmicaAbstract: Quantum heat engines are devices comprised of one or more quantum systems operating cyclically between two heat reservoirs to produce work. Although the laws of quantum mechanics rule these engines, this does not imply that these machines function in a quantum fashion, or exhibit a quantum advantage. Indeed, quantum heat engines have shown a remarkable similarity with classical models, thus raising the question of \emph{what is ''quantum'' in quantum thermodynamics}. With the help of the P-representability concept, an expression called classicality function is introduced - allowing us to quantify the degree of non-classicality of a bosonic system. As an illustration, we consider a quantum heat engine consisting of a single bosonic mode as a working substance coupled to a cold thermal and hot squeezed thermal bath. For two specific protocols, i.e., a quantum Otto and a generalized cycle, we show that the non-classicality is a resource that can be used to perform thermodynamical tasks more efficiently. Furthermore, we present a theoretical framework to study how quantum features, such as entanglement or coherence, affect the classical formulations of thermodynamic axiomsMestradoFísicaMestre em Física1765874/2018CAPE

Ultra-fast Kinematic Vortices in Mesoscopic Superconductors: The Effect of the Self-Field

Within the framework of the generalized time-dependent Ginzburg-Landau equations, we studied the influence of the magnetic self-field induced by the currents inside a superconducting sample driven by an applied transport current. The numerical simulations of the resistive state of the system show that neither material inhomogeneity nor a normal contact smaller than the sample width are required to produce an inhomogeneous current distribution inside the sample, which leads to the emergence of a kinematic vortex-antivortex pair (vortex street) solution. Further, we discuss the behaviors of the kinematic vortex velocity, the annihilation rates of the supercurrent, and the superconducting order parameters alongside the vortex street solution. We prove that these two latter points explain the characteristics of the resistive state of the system. They are the fundamental basis to describe the peak of the current-resistance characteristic curve and the location where the vortex-antivortex pair is formed.Comment: 9 pages, 6 figures. Accepted for publication in Scientific Report

Fluctuation-dissipation relations for thermodynamic distillation processes

The fluctuation-dissipation theorem is a fundamental result in statistical physics that establishes a connection between the response of a system subject to a perturbation and the fluctuations associated with observables in equilibrium. Here we derive its version within a resource-theoretic framework, where one investigates optimal quantum state transitions under thermodynamic constraints. More precisely, we first characterise optimal thermodynamic distillation processes, and then prove a relation between the amount of free energy dissipated in such processes and the free energy fluctuations of the initial state of the system. Our results apply to initial states given by either asymptotically many identical pure systems or arbitrary number of independent energy-incoherent systems, and allow not only for a state transformation, but also for the change of Hamiltonian. The fluctuation-dissipation relations we derive enable us to find the optimal performance of thermodynamic protocols such as work extraction, information erasure and thermodynamically-free communication, up to second-order asymptotics in the number $N$ of processed systems. We thus provide a first rigorous analysis of these thermodynamic protocols for quantum states with coherence between different energy eigenstates in the intermediate regime of large but finite $N$.Comment: 31 pages, 7 figures. Published versio

Machine classification for probe-based quantum thermometry

We consider probe-based quantum thermometry and show that machine classification can provide model-independent estimation with quantifiable error assessment. Our approach is based on the k-nearest-neighbor algorithm. The machine is trained using data from either computer simulations or a calibration experiment. This yields a predictor which can be used to estimate the temperature from new observations. The algorithm is highly flexible and works with any kind of probe observable. It also allows to incorporate experimental errors, as well as uncertainties about experimental parameters. We illustrate our method with an impurity thermometer in a Bose-gas, as well as in the estimation of the thermal phonon number in the Rabi model