Physics
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Item Photon Thermalization in Driven Open Quantum Systems(2018) Wang, Chiao-Hsuan; Taylor, Jacob M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Light is a paradigmatic quantum field, with individual excitations---photons---that are the most accessible massless particles known. However, their lack of mass and extremely weak interactions mean that typically the thermal description of light is that of blackbody radiation. As the temperature of the light decreases, the overall number of photons approaches zero. Therefore, efforts for quantum optics and optical physics have mostly focused on driving systems far from equilibrium to populate sufficient numbers of photons. While lasers provide a severe example of a nonequilibrium problem, recent interests in the near-equilibrium physics of so-called photon gases, such as in Bose condensation of light or in attempts to make photonic quantum simulators, suggest one re-examine near-equilibrium cases. In this thesis, we consider peculiar driven open quantum system scenarios where near-equilibrium dynamics can lead to equilibration of photons with a finite number, following a thermal description closer to that of an ideal gas than to blackbody radiation. Specifically, we show how laser cooling of a well-isolated mechanical mode or atomic motion can provide an effective bath which enables control of both the chemical potential and temperature of the resulting grand canonical ensemble of photon. We then theoretically demonstrate that Bose condensation of photons can be realized by cooling an ensemble of two-level atoms inside a cavity. Finally, we find that the engineered chemical potential for light not only admits future applications in many-body quantum simulations, facilitates preparation of near-equilibrium photonic states, but also enables an analogous voltage bias for photonic circuit elements.Item Understanding Phase Transitions, Symmetry Breaking, and Interaction-Enhanced Sensing in Optomechanical and Cold Atomic Systems(2018) Ragole, Stephen; Taylor, Jacob M; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We focus on the interaction of light and matter in atomic and optomechanical systems. These highly controllable and engineerable systems present access to new regimes and research opportunities that often do not exist outside the laboratory. As such, they frequently depart from more commonplace systems which are well understood. We extend our understanding of thermodynamic phase transitions, spontaneous symmetry breaking, and quantum-enhanced sensing to new regimes. Traditionally, phase transitions are defined in thermodynamic equilibrium. However, inspired by the success of the phase transition paradigm in non-equilibrium fields, we derive an effective thermodynamics for the mechanical excitations of an optomechanical system. Noting the common frequency separation between optical and mechanical components, we study the dynamics of the mechanical modes under the influence of the steady state of the optical modes. We identify a sufficient set of constraints which allow us to define an effective equilibrium for the mechanical system. We demonstrate these constraints by studying the buckling transition in an optomechanical membrane-in-the-middle system, which spontaneously breaks a parity symmetry. Having established a thermodynamic limit, we characterize the nature of the phase transition, which can change order based on system parameters. We extend our framework, proposing an photonic systems which realizes an SO(N) symmetry breaking transition of the same nature as the membrane-in-the-middle system. While we have treated these systems in the classical limit, their open nature has pronounced effects when other noise sources are suppressed. We study the canonical optomechanical system to unravel the origin of the semiclassical force and potential on the mechanics. We find that this force, while conservative with respect to the mechanics, deeply depends on the quantum back-action due to photon loss from the cavity. Additionally, we study the ability of cold atoms to sense rotation. We consider bosonic atoms confined to a one-dimensional ring. Employing Luttinger liquid theory to study the excitations, we find that in the strongly-repulsive regime, atomic currents can be manipulated and superposed by controlling a laser barrier. These superpositions provide a Heisenberg-limited rotation sensing method. When we include noise, the precision is reduced, but the performance still surpasses the standard quantum limit. We comment on the applicability of such a sensor for inertial sensing.