Tag: Physics Colloquium

Physics Seminar: Negative refractive index for graphene and surface Plasmon instability for hybrid structures

Presented by: Distinguished Professor Godfrey Gumbs

Femtosecond and subfemtosecond time scales typically rule electron dynamics at metal surfaces. Recent advances in experimental techniques allow the experimental study of such dynamics. In this talk, we shall analyze electron dynamics at surfaces and nanostructures with emphasis on screening times, spin dependence of charge transfer of adsorbates and smaller system sizes. We will discuss the effect of energy gaps on possible “Veselago lenses” for completely flat graphene sheets. We will also discuss how Plasmon instabilities may be exploited for tunable radiation generation which may be employed in detectors.

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Physics Seminar: Prof. William Wootters:
Why does nature like the square root of negative one?

[icon name=”map-marker” class=”” unprefixed_class=””] Place: Namm 823
[icon name=”calendar” class=”” unprefixed_class=””] Date: Thursday, March 31 2016

[icon name=”clock-o” class=”” unprefixed_class=””] Time: 12:00 PM

 

Presented by Prof. William Wootters
Faculty and students are welcome.

 

Abstract:

Quantum mechanics is a probabilistic theory, but the way we compute probabilities in quantum mechanics is quite different from what one would expect from, say, rolling dice or tossing coins.  To get a quantum probability, we first compute a complex-valued probability amplitude and then square its magnitude.  I begin this talk by looking for a deeper explanation of the appearance of probability amplitudes, or “square roots of probability,” in the physical world.  It turns out that one can find a potential explanation—it is based on a principle of optimal information transfer—but the argument works only if the square roots are real rather than complex.  I then discuss a few of the ideas people have put forward to try to understand why nature favors complex amplitudes.  At present no such idea has gained wide acceptance, but the effort to answer this question has produced insights into the structure of quantum theory.    

Physics Seminar: Chiral Bose and Fermi phases in orbital optical lattices

[icon name=”map-marker” class=”” unprefixed_class=””] Place: Namm 823
[icon name=”calendar” class=”” unprefixed_class=””] Date: Thursday, December 3 at

[icon name=”clock-o” class=”” unprefixed_class=””] Time: 12:00 PM

Presented by Prof. W. Vincent Liu

Faculty and students are welcome, light refreshments will be served.

Abstract:
When interacting ultracold atoms are loaded into the metastable but long lived higher orbital excited bands of an optical lattice, would it be possible for the atoms to exhibit conceptually novel phases that have no prior analogue from the well-known past condensed matter models? In this talk, I will report some of our recent findings when exploiting symmetries, quantum phases, and topology beyond natural conditions in such artificial quantum orbital systems.

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Physics Seminar: Quantum phases in a chain of coupled fluxonium qubits

[icon name=”map-marker” class=”” unprefixed_class=””] Place: Namm 823
[icon name=”calendar” class=”” unprefixed_class=””] Date: Thursday, November 12

[icon name=”clock-o” class=”” unprefixed_class=””] Time: 12:00 PM

 

Presented by Dr. Richard Brierley

Faculty and students are welcome, light refreshments will be served.

Abstract:
Recent progress in the field of superconducting circuits has led to the development of many different types of qubit that can be precisely controlled and monitored. The techniques of circuit QED, where qubits are strongly coupled to microwave cavities, enable direct access to excitations and control of dissipation, making large arrays of superconducting qubits a promising platform for studying quantum many-body physics.

I will present a theoretical study of an array of “fluxonium” qubits, consisting of a chain of Josephson junctions that are coupled to ground via linear inductors. When an external magnetic field is applied, the competition between inductive and Josephson energies leads to a rich classical phase diagram, including a devil’s staircase of phases. The inclusion of quantum effects results in regions of Luttinger liquid behavior and phase transitions in the commensurate-incommensurate, Kosterliz-Thouless and Ising universality classes. As well as discussing the ground states, I will show how the different phases may be experimentally identified through their excitation spectra.

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Mathematics and Physics Colloquium: Symmetric Class-0 Subgraphs and Forbidden Subgraphs

[icon name=”map-marker” class=”” unprefixed_class=””] Place: Namm 720
[icon name=”calendar” class=”” unprefixed_class=””] Date: Thursday October 22, 2015

[icon name=”clock-o” class=”” unprefixed_class=””] Time: 12:45 p.m.

Presented by Prof. Eugene Fiorini
Faculty and students are welcome, light refreshments will be served.

Abstract:
Competition graphs and graph pebbling are two examples of graph theoretical-type games played on a graph under well-defined conditions. In the case of graph pebbling, the pebbling number pi(G) of a graph G is the minimum number of pebbles necessary to guarantee that, regardless of distribution of pebbles and regardless of the target vertex, there exists a sequence of pebbling moves that results in placing a pebble on the target vertex. A class-0 graph is one in which the pebbling number is the order of the graph, pi(G)=|V(G)|. This talk will consider under what conditions the edge set of a graph G can be partitioned into k class-0 subgraphs, k a positive integer. Furthermore, suppose D is a simple digraph with vetex set V(D) and edge set E(D). The competition graph G(V(G),E(G)) of D is defined as a graph with vertex set V(G)=V(D) and edge vw in E(G) if and only if for some vertex u in V, there exist directed edges (u,v) and (u,w) in E(D). This talk will present some recent results on forbidden subgraphs of a family of competition graphs.

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Physics Seminar: Pattern Formation and Strong Nonlinear Interactions in Exciton-Polariton Condensates

[icon name=”map-marker” class=”” unprefixed_class=””] Place: Namm 823
[icon name=”calendar” class=”” unprefixed_class=””] Date: Thursday October 22, 2015

[icon name=”clock-o” class=”” unprefixed_class=””] Time: 12:00 p.m.

Presented by Dr. Li Ge
Faculty and students are welcome, light refreshments will be served.

Abstract:
Exciton-polaritons generated by light-induced potentials can spontaneously condense into macroscopic quantum states that display nontrivial spatial and temporal density modulation. While these patterns and their dynamics can be reproduced through the solution of the generalized Gross-Pitaevskii equation (GPE), a predictive theory of their thresholds, oscillation frequencies, and multi-pattern interactions has so far been lacking. Here we represent such an approach based on current-carrying quasi-modes of the non-Hermitian potential induced by the pump. The presented theory allows us to capture the patterns formed in the steady-state directly and account for the nonlinearities exactly in GPE. We find a simple but powerful expression for thresholds of condensation and the associated frequencies of oscillations, quantifying the contribution of particle formation, leakage, and interactions. We also show that the evolution of the condensate with increasing pump strength is strongly geometry dependent and can display contrasting features such as enhancement or reduction of the spatial localization of the condensate. The comparison with two independent experiments will be discussed.

[icon name=”file-pdf-o” class=””] Click to view poster.