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Events on Thursday, March 10th, 2016

R. G. Herb Condensed Matter Seminar
Phonon-mediated decay of singlet-triplet qubits in double quantum dots in GaAs/AlGaAs and Si/SiGe heterostructures
Time: 10:00 am
Place: Chamberlin 5310
Speaker: Viktoriia Kornich, University of Basel
Abstract: We study theoretically the phonon-induced relaxation and decoherence times of singlet-triplet qubits in lateral double quantum dots (DQDs) in GaAs/AlGaAs and Si/SiGe heterostructures. We show that the two-phonon processes can dominate over the one-phonon processes even at rather low temperature, e.g. for the decoherence time at 50 mK for biased GaAs/AlGaAs DQDs. We find that when the DQD is unbiased the qubit lifetimes are orders of magnitude longer than for the case of biased DQD. The lifetimes of the qubits in Si/SiGe DQDs are remarkably longer than the ones in GaAs/AlGaAs DQDs, and following interest in S-T_- qubits we consider the phonon-induced decoherence and relaxation particularly in the region of the singlet-triplet anticrossing. We show that the hybridization of the singlet state plays a crucial role. Among other parameters of the system, we study the effect of micromagnet-induced magnetic field gradient on the qubit, which reveals an non-trivial behavior of the relaxation and decoherence times as a result of interplay between one-phonon and two-phonon processes. We suggest how our theory can be tested experimentally.
Host: Vavilov
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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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Astronomy Colloquium
"Toward a physically-predictive theory of galaxy formation: resolved stellar and black hole feedback"
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies 3:30 PM, Talk at 3:45 PM
Speaker: Claude-Andre Faucher-Giguere, Northwestern University
Abstract: Over the past few years, cosmological hydrodynamic simulations have begun to produce galaxy populations with properties that agree broadly with observations. The successes of most existing simulations however rely on carefully tuning parameters of sub-grid models for core physical processes such as star formation, stellar feedback, and supermassive black holes. I will describe a research program that aims to greatly improve the predictive power of galaxy formation models by directly resolving some of the key physical processes in the ISM of galaxies and developing approximations for unresolved processes that are directly calibrated using small-scale calculations. This approach largely eliminates the need for tunable parameters. I will present some key results from the FIRE ("Feedback In Realistic Environments") cosmological simulations developed using this approach, including on the physics shaping the galaxy stellar mass function, the generation of galactic winds, the chemical enrichment of galaxies, and observational diagnostics of circum-galactic gas flows. I will conclude by outlining our on-going work to model supermassive black hole growth and feedback, from galactic nucleus to cosmological scales.<br>
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