DRUM Community: Physicshttp://hdl.handle.net/1903/22692014-04-18T13:34:48Z2014-04-18T13:34:48ZPhotocathodes: Mapping, Controlled Porosity, Cesium, and GoldRiddick, Blake Cutlerhttp://hdl.handle.net/1903/149442014-02-13T03:30:42Z2013-01-01T00:00:00ZTitle: Photocathodes: Mapping, Controlled Porosity, Cesium, and Gold
Authors: Riddick, Blake Cutler
Abstract: Photocathodes play a critical role in a large variety of applications, from detectors in medical imaging to electron beam sources for fundamental science. Via the photoelectric effect, photocathodes emit electrons in response to light of sufficient energy. Three important characteristics of photocathodes are quantum efficiency (the fraction of incident photons that generates emitted electrons), lifetime (how long the cathodes are operational), and emission uniformity. Coating a photocathode with an atomic layer of cesium dramatically improves quantum efficiency, but the inherent fragility of this layer worsens lifetime. The design and testing of a cesium rejuvenation system which prolongs lifetime will be presented, and a controlled porosity design which could improve emission uniformity will be discussed. A new method of mapping quantum efficiency will be presented. The initial results of testing cesium auride will be discussed, as they show the cathodes have surprisingly high quantum efficiency and the potential for very long lifetime.2013-01-01T00:00:00ZMulti-junction effects in dc SQUID phase qubitsCooper, Benjamin Kevinhttp://hdl.handle.net/1903/149322014-02-12T03:31:52Z2013-01-01T00:00:00ZTitle: Multi-junction effects in dc SQUID phase qubits
Authors: Cooper, Benjamin Kevin
Abstract: I discuss experimental and theoretical results on an LC filtered dc SQUID phase qubit. This qubit is an asymmetric aluminum dc SQUID, with junction critical currents 1.5 and 26.8 μA, on a sapphire substrate. The layout differs from earlier designs by incorporating a superconducting ground plane and weakly coupled coplanar waveguide microwave drive line to control microwave-qubit coupling.
I begin with a discussion of quantizing lumped element circuit models. I use nodal analysis to construct a 2d model for the dc SQUID phase qubit that goes beyond a single junction approximation. I then discuss an extension of this ``normal modes'' SQUID model to include the on-chip LC filter with design frequency ∼ 180 MHz. I show that the filter plus SQUID model yields an effective Jaynes-Cummings Hamiltonian for the filter-SQUID system with coupling g / 2 π ∼ 32 MHz.
I present the qubit design, including a noise model predicting a lifetime T<sub>1</sub> = 1.2 μs for the qubit based on the design parameters. I characterized the qubit with measurements of the current-flux characteristic, spectroscopy, and Rabi oscillations. I measured T<sub>1</sub> = 230 ns, close to the value 320 ns given by the noise model using the measured parameters. Rabi oscillations show a pure dephasing time T<sub>φ</sub> = 1100 ns. The spectroscopic and Rabi data suggest two-level qubit dynamics are inadequate for describing the system. I show that the effective Jaynes-Cummings model reproduces some of the unusual features.2013-01-01T00:00:00ZNew insights from large Nc QCDKrejcirik, Vojtechhttp://hdl.handle.net/1903/149282014-02-12T03:31:43Z2013-01-01T00:00:00ZTitle: New insights from large Nc QCD
Authors: Krejcirik, Vojtech
Abstract: In this dissertation, three problems--one in meson physics, one in baryon physics, and one in nuclear physics--are analyzed in the large Nc limit of quantum chromodynamics. It is shown that a Hagedorn spectrum emerges naturally for mesons, i.e. that the number of mesons grows exponentially with mass; a new relation for the electromagnetic form factors of nucleon is derived in the combined large Nc and chiral limit of QCD; Nc scaling of the nucleon-nucleon scattering S-matrix is deduced as well as an exact formula for the nucleon-nucleon cross section in the leading order in 1/Nc expansion is derived.2013-01-01T00:00:00ZNonequilibrium Quantum Systems: Fluctuations and InteractionsSubasi, Yigithttp://hdl.handle.net/1903/149272014-02-12T03:31:42Z2013-01-01T00:00:00ZTitle: Nonequilibrium Quantum Systems: Fluctuations and Interactions
Authors: Subasi, Yigit
Abstract: We explore some aspects of nonequilibrium statistical mechanics of classical and quantum systems. Two chapters are devoted to fluctuation theorems which were originally derived for classical systems. The main challenge in formulating them in quantum mechanics is the fact that fundamental quantities of interest, like work, are defined via the classical concept of a phase space trajectory. We utilize the <italic>decoherent histories</italic> conceptual framework, in which classical trajectories emerge in quantum mechanics as a result of coarse graining, and provide a first-principles analysis of the nonequilibrium work relation of Jarzynski and Crooks's fluctuation theorem for a quantum system interacting with a general environment based on the <italic>quantum Brownian motion</italic> (QBM) model. We indicate a parameter range at low temperatures where the theorems might fail in their original form.
Fluctuation theorems of Jarzynski and Crooks for systems obeying classical Hamiltonian dynamics are derived under the assumption that the initial conditions are sampled from a canonical ensemble, even though the equilibrium state of an isolated system is typically associated with the microcanonical ensemble. We address this issue through an exact analysis of the <italic>classical Brownian motion</italic> model. We argue that a stronger form of <italic>ensemble equivalence</italic> than usually discussed in equilibrium statistical mechanics is required for these theorems to hold in the infinite environment limit irrespective of the ensemble used, and proceed to prove it for this model. An exact expression for the probability distribution of work is obtained for finite environments.
Intuitively one expects a system to relax to an equilibrium state when brought into contact with a thermal environment. Yet it is important to have rigorous results that provide conditions for equilibration and characterize the equilibrium state. We consider the dynamics of open quantum systems using the Langevin and master equations and rigorously show that under fairly general conditions quantum systems interacting with a heat bath relax to the equilibrium state defined as the reduced thermal state of the system plus environment, even in the strong coupling regime. Our proof is valid to second-order in interaction strength for general systems and exact for the linear QBM model, for which we also show the equivalence of multi-time correlations.
In the final chapter we give a sampling of our investigations into <italic>macroscopic quantum phenomena</italic>. We work out in detail a specific example of how and under what conditions the center of mass (CoM) coordinate of a macroscopic object emerges as the relevant degree of freedom. Interaction patterns are studied in terms of the couplings they induce between the CoM and relative coordinates of two macroscopic objects. We discuss the implications of these interaction patterns on macroscopic entanglement.2013-01-01T00:00:00Z