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This Week at Physics

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Events on Thursday, April 3rd, 2014

R. G. Herb Condensed Matter Seminar
Measuring topological transitions in superconducting qubits
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Michael Kolodrubetz, Boston University
Abstract: The field of topological insulators sprung from the realization that in the presence of spin-orbit coupling, non-interacting electrons can have a band structure that non-trivially wraps the first Brillouin zone. From the gauge-invariant Berry curvature that locally defines the geometry of this wrapping, one can define an integer topological invariant – the Chern number – from which all other invariants derive. We investigate the Berry curvature and Chern number of an even simpler case: single and double spin-1/2 systems (qubits) in a rotating magnetic field. We show that these simple systems undergo topological transitions of their Chern number, which for the case of the single qubit can be directly mapped to the topological transitions of the Haldane model of graphene. Furthermore, we experimentally demonstrate such a topological transition in a single superconducting qubit, measuring the Berry curvature as a leading order correction to linear response. We then generalize the methods to two-qubit systems, where we experimentally measure the topological phase diagram and demonstrate interaction-driven topological transitions.
Host: Vavilov
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Cosmic voids as cosmological laboratories
Time: 2:00 pm
Place: 4274 Chamberlin
Speaker: Paul Sutter, IAP
Abstract: Voids are the large, underdense regions in the cosmic web, and they are potentially powerful cosmological probes due to their intimate connection to the growth of structure, their domination by dark energy, and their relative lack of systematics. I will present our latest work to identify voids in galaxy redshift surveys, our efforts to understand their fundamental nature and their connection to dark matter underdensities, and an overview of many diverse cosmological applications, including gravitational lensing, the ISW effect, and the Alcock-Paczynski effect.
Host: Peter Timbie
Poster: http://www.physics.wisc.edu/twap/posters/2014/3164.pdf
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Astronomy Colloquium
The Gaseous Environments of Galaxies: Toward Lower Halo Masses and Absorption Morphology
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Kate Rubin, Harvard-Smithsonian Center for Astrophysics
Abstract: The environments extending several hundred kiloparsecs from galaxies contain the fuel that feeds galactic star formation, and act as the reservoir into which ejecta from stellar and AGN feedback are driven. Observations of the cool hydrogen and metal content of these regions (i.e., the circumgalactic medium, or CGM) can therefore provide incisive tests of our understanding of these processes. I will briefly discuss current constraints on the content of the CGM around massive, &amp;gt; L* galaxies from z~2 to ~0. I will then describe a new technique that pinpoints much fainter, sub-luminous systems at z~2, allowing us to probe the gas in their surroundings in absorption toward background QSOs for the first time. I will also discuss prospects for new constraints on the sizes of CGM absorbers, and provide a first glimpse into the small-scale distribution of this diffuse material. Such measurements represent important steps toward a complete, detailed empirical picture of the CGM.

Host: Aleks Diamond Stanic
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