Physics
Permanent URI for this communityhttp://hdl.handle.net/1903/2269
Browse
3 results
Search Results
Item Foundational Theoretical Issues of Quantum Heat Engines and Hot Entanglement(2022) Arisoy, Onat; Hu, Bei-Lok; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study open quantum systems in the contexts of quantum heat engines, refrigerators and quantum entanglement in systems in contact with high temperature reservoirs. Emphasizing the underlying theoretical foundations rather than practical aspects such as enhancement in efficiency of engines and refrigerators or in entanglement measures for a particular set of system and bath parameters, this work focuses on some important aspects of open quantum systems and quantum thermodynamics and provide in-depth analysis of them using relatively simple yet robust models in non- equilibrium statistical mechanics. These examples include a refrigerator for quantum many-body systems in the Markovian regime, a single harmonic oscillator quantum Otto cycle with its generalization to squeezed thermal baths and the entanglement dynamics of two coupled harmonic oscillators each having its own separate thermal baths in the non-Markovian regime. The investigation of these setups in a unified context in this dissertation also brings up the discussion on the validity of Markovian approximation for open quantum systems and the qualitative differences in Markovian versus non-Markovian open system dynamics, which is addressed on multiple occasions throughout the cases we study. Our analysis of quantum Otto cycle with squeezed thermal reservoirs show that the efficiency of the cycle does not change due to the squeezing in the bath in contrast to previous works studying the same cycle restricted by Markovian assumptions. In our investigation of the effects of time-dependent coupling in a system of two harmonic oscillators with two separate baths in both Markovian and non-Markovian regimes, we find that the driving-induced instability of the solutions of the Langevin equations of the oscillator system is necessary to sustain entanglement at late times with hot reservoirs which mayrender hot entanglement untenable. The effects of Markovianity/non-Markovianity, non-thermal reservoirs, contact with multiple reservoirs and time-dependent system Hamiltonians in quantum thermodynamics are addressed in this thesis.Item Nonequilibrium Dynamics in Open Quantum Systems(2019) Young, Jeremy; Rolston, Steven L; Gorshkov, Alexey V; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to the variety of tools available to control atomic, molecular, and optical (AMO) systems, they provide a versatile platform for studying many-body physics, quantum simulation, and quantum computation. Although extensive efforts are employed to reduce coupling between the system and the environment, the effects of the environment can never fully be avoided, so it is important to develop a comprehensive understanding of open quantum systems. The system-environment coupling often leads to loss via dissipation, which can be countered by a coherent drive. Open quantum systems subject to dissipation and drive are known as driven-dissipative systems, and they provide an excellent platform for studying many-body nonequilibrium physics. The first part of this dissertation will focus on driven-dissipative phase transitions. Despite the nonequilibrium nature of these systems, the corresponding phase transitions tend to exhibit emergent equilibrium behavior. However, we will show that in the vicinity of a multicritical point where multiple phase transitions intersect, genuinely nonequilibrium criticality can emerge, even though the individual phase transitions on their own exhibit equilibrium criticality. These nonequilibrium multicritical points can exhibit a variety of exotic phenomena not possible for their equilibrium counterparts, including the emergence of complex critical exponents, which lead to discrete scale invariance and spiraling phase boundaries. Furthermore, the Liouvillian gap can take on complex values, and the fluctuation-dissipation theorem is violated, corresponding to an effective temperature which gets "hotter" and "hotter" at longer and longer wavelengths. The second part of this dissertation will focus on Rydberg atoms. In particular, we study how the spontaneous generation of contaminant Rydberg states drastically modifies the behavior of a driven-dissipative Rydberg system due to the resultant dipole-dipole interactions. These interactions lead to a complicated competition of both blockade and anti-blockade effects, leading to strongly enhanced Rydberg populations for far-detuned drive and reduced Rydberg populations for resonant drive.Item Classical Analogies in the Solution of Quantum Many-Body Problems(2017) Keser, Aydin Cem; Galitski, Victor M.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We consider three quantum many-body systems motivated by recent developments in condensed matter physics, namely topological superconductivity, strongly interacting Bose-Einstein condensates and many-body localization with periodically driven systems. In each of the three problems, an analogy with classical mechanics is employed in the solution of the problem and the interpretation of results. These analogies, in addition to facilitating the solution, illustrate how unique features of classical mechanics or macroscopic phenomena such as macroscopic order parameter and observables, hydrodynamics, spacetime curvature, noise and dissipation, chaos and delocalization emerge out of quantum mechanics. The three problems we study are as follows. In the 1st problem, we use quasiclassical methods of superconductivity to study the superconducting proximity effect from a topological p-wave superconductor into a disordered quasi-one-dimensional metallic wire. We demonstrate that the corresponding Eilenberger equations with disorder reduce to a closed nonlinear equation for the superconducting component of the matrix Green's function. Remarkably, this equation is formally equivalent to a classical mechanical system (i.e., Newton's equations), with the Green's function corresponding to a coordinate of a fictitious particle and the coordinate along the wire corresponding to time. This mapping allows us to obtain exact solutions in the disordered nanowire in terms of elliptic functions. A surprising result that comes out of this solution is that the p-wave superconductivity proximity induced into the disordered metal remains long range, decaying as slowly as the conventional s-wave superconductivity. It is also shown that impurity scattering leads to the appearance of a zero-energy peak. In the second problem, we consider a system of bosons in the superfluid phase. Collective modes propagating in a moving superfluid are known to satisfy wave equations in a curved spacetime, with a metric determined by the underlying superflow. We use the Keldysh technique in a curved spacetime to develop a quantum geometric theory of fluctuations in superfluid hydrodynamics. This theory relies on a ``quantized" generalization of the two-fluid description of Landau and Khalatnikov, where the superfluid component is viewed as a quasi-classical field coupled to a normal component { the collective modes/phonons representing a quantum bath. This relates the problem in the hydrodynamic limit to the \quantum friction" problem of Caldeira-Leggett type. By integrating out the phonons, we derive stochastic Langevin equations describing a coupling between the superfluid component and phonons. These equations have the form of Euler equations with additional source terms expressed through a fluctuating stress-energy tensor of phonons. Conceptually, this result is similar to stochastic Einstein equations that arise in the theory of stochastic gravity. We formulate the fluctuation-dissipation theorem in this geometric language and discuss possible physical consequences of this theory. In the third problem, we investigate dynamical many-body localization and delocalization in an integrable system of periodically-kicked, interacting linear rotors. The linear-in-momentum Hamiltonian makes the Floquet evolution operator analytically tractable for arbitrary interactions. One of the hallmarks of this model is that depending on certain parameters, it manifests both localization and delocalization in momentum space. We present a set of \emergent" integrals of motion, which can serve as a fundamental diagnostic of dynamical localization in the interacting case. We also propose an experimental scheme, involving voltage-biased Josephson junctions, to realize such many-body kicked models.