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This Week at Physics

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Events on Thursday, October 26th, 2017

R. G. Herb Condensed Matter Seminar
Watching a quantum system: How to continuously measure a superconducting qubit
Time: 10:00 am
Place: Chamberlin 53100
Speaker: Justin Dressel, Chapman University
Abstract: It has recently become experimentally possible to monitor the energy levels of a superconducting transmon qubit continuously in time using microwave fields. Such measurements weakly perturb the qubit per unit time, lead to a competition between unitary Hamiltonian dynamics and non-unitary collapse dynamics. I review several subtleties about modeling this measurement process, and discuss several recent achievements made in collaboration with the Siddiqi laboratory at UC Berkeley. Topics include simultaneous measurements of multiple non-commuting observables, the active use of the quantum Zeno effect with a moving measurement basis for qubit control, and subtle aspects about the information content contained in the collected stochastic readout.
Host: Vavilov
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Astronomy Colloquium
The Plasma Physics of TeV Blazars
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies at 3:30 PM. Talk begins at 3:45 PM
Speaker: Phil Chang, UW - Milwaukee
Abstract: Constraints on the primordial intergalactic magnetic field from the non-observation of inverse Compton cascades around extragalactic very high energy sources, i.e., the TeV blazars, assume that inverse Compton scattering is the dominant physical mechanism by which dilute ultrarelativistic pair beams lose their energy. Over the last few years, we have considered the effect of plasma instabilities on these ultrarelativistic beams. We argue that the linear growth rate of these instabilities, and in particular the oblique instability, are so fast that these instabilites may dominate the cooling of these pair beams leading to an order of magnitude or more suppression in the inverse Compton cascade. We review the relevant physics of these
plasma instabilities and discuss the linear instability of these pair beams. We also discuss recent work on the various nonlinear aspects of this instability and the effect of density gradients on the instability. We highlight the effect of this instability on the constraints of the intergalactic magnetic field, arguing that these constraints are precluded in the presences of these instabilities. We also discuss the implication of these instabilities on the population of TeV blazars, and the intergalactic gamma ray background. Finally, we close with a discussion on the effect of these extra blazar heating on cosmological structure formation, in particular, the<br>
temperature-overdensity profile and the Lyman-alpha forest.<br><br>
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