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This Week at Physics

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Events During the Week of October 1st through October 8th, 2017

Monday, October 2nd, 2017

Plasma Physics (Physics/ECE/NE 922) Seminar
FLASH simulations of a magnetized turbulence experiment at the National Ignition Facility
Time: 12:00 pm
Place: Chamberlin 2241
Speaker: Dr. Scott Feister, Flash Center for Computational Science, University of Chicago
Abstract: The National Ignition Facility (NIF), the world’s most energetic laser, is more than an inertial confinement fusion platform – it can drive extreme plasma conditions for laboratory astrophysics. The University of Chicago and its international team of collaborators won allocation of several NIF shots, spaced over a few years, to study the development of magnetized turbulence and turbulent dynamo. By giving validated simulations a central role in the experimental design process, we have endeavored to "not throw away our shot." The 3D MHD code FLASH was validated in a range of smaller experiments, then it was used to simulate our experimental ideas from start (laser interaction) to finish (plasma diagnostics). We have replicated plasma diagnostics within FLASH to make one-to-one comparisons with experimental data, and make choices about diagnostic timings and exposures. Our simulations have also informed many aspects of the experimental target design and laser drive as implemented at NIF. These simulations show Biermann battery magnetic fields seeded in the laser interaction and amplified in a central, highly-turbulent dynamo region.
Host: Stanislav Boldyrev
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Council Meeting
Council Meeting
Time: 4:00 pm
Place: 2314 Chamberlin Hall
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Tuesday, October 3rd, 2017

Chaos & Complex Systems Seminar
Systematic misperceptions of 3D motion explained by Bayesian inference
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Bas Rokers, UW Department of Psychology
Abstract: Over the years, a number of surprising, but seemingly unrelated errors in 3D motion perception have been reported. Given the relevance of accurate motion perception to our everyday life, it is important to understand the cause of these perceptual errors.

We considered that these perceptual errors might arise as a natural consequence of estimating motion direction given sensory noise and the geometry of 3D viewing. We characterized the retinal motion signals produced by objects moving along arbitrary trajectories through three dimensions and developed a Bayesian model of perceptual inference. The model predicted a number of known errors, including a lateral bias in the perception of motion trajectories, and a dependency of this bias on stimulus contrast and viewing distance. The model also predicted a number of previously unknown errors, including a dependency of perceptual bias on eccentricity, and a surprising tendency to misreport approaching motion as receding and vice versa.

We then used standard 3D displays as well as a virtual reality (VR) headsets to test these predictions in naturalistic settings, and established that people make the predicted errors.

In sum, we developed a quantitative model of 3D motion perception and provided a parsimonious account for a range of systematic perceptual errors in naturalistic environments.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Topics in Axion Cosmology
Time: 3:30 pm
Place: 5280 Chamberlin
Speaker: Andrew Long, KICP/University of Chicago
Abstract: (Which topics? The interesting ones!) Light pseudo-Goldstone bosons arise in many compelling models of particle physics and string theory. These particles can modify cosmology in interesting, surprising, and testable ways. In this talk I will discuss three topics. First I’ll argue that the dynamics of an axion field during inflation can give rise to the matter / antimatter asymmetry of the universe via the production of helical magnetic fields (which persist today and might be detectable!). Second, I’ll discuss what goes wrong when you try to implement the same idea with chiral gravitational waves instead of magnetic fields. Finally, I’ll talk about new phenomena that arise in multi-axion models, such as the recent proposed `clockwork’ axion, that have a large hierarchy between the scale of PQ-breaking and the axion decay constant.
Presentation: Andrew_Long--UW.pdf
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PUMP: Perspective Undergrads Majoring in Physics
PUMP: Perspective Undergrads Majoring in Physics
Time: 4:30 pm
Place: 2241 Chamberbelin
Abstract: Are you interested in the Physics Undergraduate Major? Come to PUMP!

Topics of discussion include: why you should major in physics, what you can do with a physics degree, physics major requirements,and research, tutoring, and mentoring opportunities available.

Meeting is informal, no RSVP needed.
Presentation: poster2017.pdf
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Wednesday, October 4th, 2017

Atomic Physics Seminar
Stark-tuned Forster resonances in Rydberg atoms and their application for quantum information
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Dr. Ilya Beterov, Institute of Semiconductor Physics Novosibirsk, Russia
Abstract: We present results of theoretical and experimental studies of Stark-tuned Forster resonances in Rydberg atoms. Two trapped<br>
atoms are excited into Rydberg states which are then tuned by the external electric field midway between two other Rydberg states of the opposite parity. The collective two-atom states are coupled by dipole-dipole interaction. We have proposed schemes of two-qubit gates using adiabatic passage of Forster resonances in time-dependent electric field. We have shown that radiofrequency electric fields can induce additional "inaccessible" resonances which are useful both for coherent coupling and for adiabatic passage. We have experimentally observed Borromean three-body Forster resonances for three interacting Rydberg atoms.
Host: Saffman
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Department Meeting
Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, October 5th, 2017

R. G. Herb Condensed Matter Seminar
Quantum dot chains as emulators of topological superconductors
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Sergey Frolov, University of Pittsburgh
Abstract: Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the magnetic-field driven topological phase transition. Specifically, we map out the phase diagram of the topological phase in the space of Zeeman energy and chemical potential, and investigate the apparent closing and re-opening of the superconducting gap. We investigate how the topological superconducting phase would manifest in finite size systems, by electrostatically splitting the wire into segments of varied length. By chaining up several segments of a nanowire, we are realizing a quantum simulator of the Kitaev chain with tunable on-site energies and couplings between the sites, a step towards quantum simulation with semiconductor nanostructures.
Host: Alex Levchenko
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Friday, October 6th, 2017

Graduate Introductory Seminar
Experimental Condensed Matter Physics
Time: 12:00 pm
Place: 4274 Chamberlin Hall
Speaker: Brar, Eriksson, McDermott, Onellion, Rzchowski, Winokur
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Physics Department Colloquium
Exciton Condensates are Super!
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Allan MacDonald , The University of Texas at Austin
Abstract: Electronic systems can have a type of order in which coherence is spontaneously established between two distinct groups of electrons. So far this (particle-hole or exciton condensate) type of order has been conclusively demonstrated only in double-layer two-dimensional electron gas systems, and only in certain strong magnetic field limits. I will review some of the surprising superfluid transport effects that have been observed in double-layer exciton condensates, and speculate on the possibility of engineering similar effects at room temperature either by engineering favorable conditions in van der Waals material bilayers or by designing ferromagnetic materials with appropriate properties.
Host: Robert Joynt
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