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This Week at Physics

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Events During the Week of January 22nd through January 29th, 2017

Monday, January 23rd, 2017

Plasma Physics (Physics/ECE/NE 922) Seminar
Divertor Transport Physics in the LargeHelical Device
Time: 12:00 pm
Place: 2317 Engineering Hall
Speaker: Dr. Masahiro Kobayashi, National Institute for Fusion Science, Japan
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Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:15 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Amol Upadhye (aupadhye@wisc.edu).
Host: Amol Upadhye
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Tuesday, January 24th, 2017

Atomic Physics Seminar
A Brief History of Time(keeping): Metrology and quantum simulation with optical lattice clocks
Time: 12:00 pm
Place: 5280 Chamberlin Hall
Speaker: Shimon Kolkowitz, JILA - University of Colorado Boulder
Abstract: Optical lattice clocks (OLCs) are now the most stable and accurate timekeepers in the world, with fractional accuracies equivalent to neither losing nor gaining a second over the entire age of the universe. This unprecedented level of metrological precision offers sensitivity to new quantum, many-body, and fundamental physics effects, opening the door to exciting and unusual applications. However, the current generation of OLCs are now approaching their classical limits, requiring quantum science techniques to reach the next frontier in clock performance.

This talk will provide an introduction to how and why time is measured, with an emphasis on OLCs and their applications. I will discuss recent progress on pushing OLCs to even greater levels of precision, as well as prospects for future improvement. I also will present results from a recent experiment in which we harnessed the precision of an OLC to simulate complex condensed matter phenomena. Finally, I will give a brief overview of potential future applications of OLCs, including gravitational wave detection, tests of general relativity, and searches for physics beyond the Standard Model.
Host: Thad Walker
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Chaos & Complex Systems Seminar
The Lyapunov exponent
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: George Hrabovsky, Madison Area Science and Technology
Abstract: The Lyapunov exponent is one of the fundamental measures of chaos. What is it? Where does it come from? What does it actually do for us? Can we predict its value? I will explore these ideas in a non-rigorous way.
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
Discovery of Higgs Boson
Time: 1:20 pm
Place: 2241 Chamberlin Hall
Speaker: Matthew F Herndon, UW Madison Department of Physics
Host: Wesley Smith
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Hunting for Sterile Neutrinos at Long and Short Baselines
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Adam Aurisano, UC Cincinnati
Abstract: The discovery that neutrinos can change flavor as they travel over long distances is one of the first indications of physics beyond the Standard Model. Long-baseline neutrino oscillation experiments, where detectors are placed far from the production point of the neutrinos, agree well with the three-flavor neutrino oscillation paradigm. However, some short-baseline experiments, where detectors are placed close to the production point of the neutrinos, show anomalous results. In particular, LSND and MiniBooNE observed excesses of electron neutrinos in muon neutrino beams which cannot be explained by three-flavor oscillations. These anomalies can be explained by the addition of new sterile neutrino flavors, but for this to be the correct explanation, long and short-baseline experiments must be consistent. If confirmed, the discovery of sterile neutrinos would be a paradigm shift in our understanding of neutrino properties and potentially of the evolution of the Universe. I will give an introduction to sterile neutrinos, present recent results from the MINOS and NOvA experiments, discuss new Deep Learning methods to improve sterile analyses, and describe prospects for the future.
Host: Sridhara Dasu
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Wednesday, January 25th, 2017

Department Meeting
Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin hall
Speaker: Albrecht Karle
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Thursday, January 26th, 2017

R. G. Herb Condensed Matter Seminar
Observation of new fractional quantum Hall states in graphene heterostructures
Time: 10:00 am
Place: 5310 Chamberlin hall
Speaker: Andrea Young, University of California Santa Barbara
Abstract: I will describe recent experiments probing the many body ground states of a new generation of ultra-clean graphene heterostructures. By replacing conventional gate metallization based on deposition of amorphous films with single crystal graphite flakes, we find that the electronic quality of graphene heterostructures can be substantially improved, rivaling that of long-studied semiconductor quantum wells. I will discuss two recent results that have emerged from these ultra-clean electronic devices. In the first part of the talk, I will describe the fate of the half filled Landau level in bilayer graphene, where different orbital Landau levels host composite fermion liquid (for N=0) and a gapped Pfaffian quantum Hall phase (for N=1). By controllably moving occupation between these two orbitals, we find that a new phase emerges, characterized by charge gap but large inter-orbital polarization. Numerical modeling suggests that the resulting phase hosts a Fermi surface of neutral excitons, and I will discuss experimental schemes to directly probe these unusual particles.

In the second part of the talk, I will describe the effects of a superlattice on the nature of the fractional quantum Hall effect. Recent theoretical work, largely motivated by efforts to engineer fractional quantum Hall states in optical lattice systems, has suggested that new kinds of fractional quantum Hall states--termed fractional Chern insulators--can exist in lattice systems with intrinsically finite bandwidth. At high magnetic field in our devices, a substrate-induced moire superlattice gives rise to a variety of Hofstadter bands with different Chern numbers. We find a wide variety of incompressible states at fractional filling of these bands characterized by fractionally quantized Hall conductance. These results demonstrate that fractional Chern insulators are indeed a generic phenomenon. Going forward, we anticipate being able to place approximate limits on the required interaction strength and bandwidth required to realize these phases.
Host: Levchenko
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Atomic Physics Seminar
From fundamental physics to aspects of photosynthesis: Controlling and studying complex quantum systems
Time: 12:00 pm
Place: 5280 Chamberlin Hall
Speaker: Boerge Hemmerling, UC Berkeley
Abstract: The answer to many scientific questions ranging from fundamental physics to aspects of photosynthesis lie in the study of quantum systems. A requirement for such studies is often to initialize the systems, manipulate them and read them out. However, many of the systems with interesting applications tend to have a complex level structure rendering these requirements difficult to meet.

In this talk, I will discuss experimental strategies to control complex ions and molecules for which standard trapping, cooling and state manipulation methods fail. In particular, I will discuss how complex ions, such as Ti+ or Fe+, can be studied and used to place limits on the temporal variation of fundamental constants. Moreover, I will present a strategy to laser cool the diatomic molecule calcium monofluoride, a precursor to produce a degenerate dipolar quantum gas. Finally, I will show how strings of ions can be used to emulate processes relevant for transport phenomena in light harvesting processes.

I will conclude with a discussion on how to control and study two further quantum systems: electrons and aluminum chloride. Electrons can be stored in a novel two-frequency Paul trap, constituting the first step towards electron quantum computing; such a trap has, moreover, the potential to advance studies on matter-antimatter asymmetries by improving antihydrogen production. Furthermore, I will explain a laser cooling scheme for aluminum chloride, a molecule with excellent prospects for generating high phase-space density clouds at ultracold temperatures to study the physics of degenerate dipolar quantum gases.
Host: thad Walker
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Multi-messenger searches for astrophysical sources at ultra-high energies
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Foteini Oikonomou, Penn State University
Abstract: The question of the origin of ultra high energy, > 10^19 eV, cosmic rays (UHECRs) remains unanswered, although experimental searches in the last decade have yielded important results, and insights about the universe at ultra-high energies. I will discuss the interpretation of the most recent measurements of the extensive air-showers produced by UHECRs, and outline current strategies aiming to answer the question of UHECR origin. Emphasis will be given to studies of UHECR arrival directions, searches for single UHECR sources through their secondary gamma-ray and neutrino signatures, and the nascent program, by the Pierre Auger Observatory, of real-time searches for transient UHE emission, as part of multi-messenger monitoring networks.
Host: Stefan Westerhoff
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Astronomy Colloquium
An X-ray View of the Dusty Universe
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies 3:30 pm, Talk at 3:45 pm
Speaker: Lia Corrales, Einstein Fello, UW Madison Astronomy Department
Abstract: A significant fraction of the heavy elements produced by stars spend some time in the interstellar medium as dust grains. These heavy metal transporters influence gas cooling during star formation, eventually becoming the seeds for planet formation. Much like quasar spectra are used to probe intergalactic gas, observations of X-ray bright Galactic compact objects can yield key insights to the mineralogy and evolution of dust grains in the Milky Way. With high resolution X-ray spectroscopy, we can directly measure the state of metals and the mineral composition of dust in the interstellar medium. In addition, dust scattering produces a diffuse halo image around bright X-ray objects, revealing information about dust grain sizes and their spatial distribution. I will review the most recent exciting dust scattering discoveries, which draw on multi-wavelength observations. Finally, I will discuss open questions regarding our X-ray view of the ISM that can be addressed with lab experiments and future X-ray observatories.<br>
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Discovering a new approach to cosmology with the Dark Energy Survey and Gravitational Waves
Time: 4:00 pm
Place: 4274 Chamberlin Hall
Speaker: Marcelle Soares-Santos, Fermi Lab
Abstract: Motivated by the exciting prospect of new wealth of information that will arise from observations of gravitational and electromagnetic radiation from the same astrophysical phenomena, the Dark Energy Survey (DES) Collaboration has performed a broad range follow-up program for LIGO/Virgo events using its Camera (DECam). In this talk, I present an overview of this effort, including results of searches for signatures of the first two LIGO-triggered binary black hole mergers in the 2015-2016 observing campaign and status of the ongoing 2016-2017 campaign. I will also discuss plans for upcoming seasons and long term prospects for this exciting emerging field: multi-messenger cosmology with gravitational waves and optical data.
Host: Sridhara Dasu
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Friday, January 27th, 2017

Atomic Physics Seminar
AMO
Ultralow-Power Nonlinear Optics using Metastable Xenon in a Cavity
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Garrett Hickman, UMBC
Abstract: Single-photon cross phase shifts and other single-photon nonlinearities have numerous applications in all-optical quantum information processing. Several groups have experimentally achieved single-photon phase shifts on the order of pi. However, nonlinearities weaker than this have important applications as well. We introduce the idea of using metastable xenon gas in a high-finesse cavity to produce weak single-photon nonlinearities. This relatively simple and robust system avoids problems associated with the accumulation of alkali atoms on mirror surfaces, and is capable of approaching the strong coupling regime of cavity quantum electrodynamics. We demonstrate the feasibility of our approach with two proof-of-principle demonstrations, by measuring absorption saturation and cross-phase modulation using a cavity of moderately high finesse F=3,000. We find that the nonlinear effects occur at ultralow input power levels, proving that the presence of the cavity strongly enhances the inherent optical nonlinearities of metastable xenon. We close our discussion by reviewing our recent progress in building an improved cavity system, which is expected to produce enhanced single photon cross phase shifts.
Host: Saffman
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Physics Department Colloquium
A New Spin on Superconductivity
Time: 3:30 pm
Place: 2241 Chamberlin hall
Speaker: Amir Yacoby, Harvard University
Abstract: Nearly a hundred years after its discovery, superconductivity remains one of the most intriguing phases of matter. In 1957 Bardeen, Cooper and Schrieffer (BCS) presented their theory of superconductivity describing this state in terms of pairs of electrons arranged in a spatially isotropic wave function with no net momentum and a spin singlet configuration. Immediately thereafter, a search began to find materials with unconventional superconductivity where pairing deviates from conventional BCS theory.

One particular class of unconventional superconductors involves pairs arranged in triplet rather than singlet configurations. Such superconductors may enable dissipationless transport of spin and may also give rise to elementary excitations that do not obey the conventional Fermi or Bose statistics but rather have non-Abelian statistics where the exchange of two particles transforms the state of the system into a new quantum mechanical state.

In this talk I will describe some of our recent experiments that explore the proximity effect between a conventional superconductor and a semiconductor with strong spin-orbit interaction. Using supercurrent interference, we show that we can tune the induced superconductivity continuously from conventional to unconventional that is from singlet to triplet. Our results open up new possibilities for exploring unconventional superconductivity as well as new ways for detecting unconventional pairing in known materials.
Host: Mark Eriksson
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