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Events on Thursday, March 14th, 2024

R. G. Herb Condensed Matter Seminar
Detecting, Studying, and Eliminating Quasiparticles in Superconducting Circuits
Time: 10:00 am - 11:00 am
Place: 5310 Chamberlin
Speaker: Eli Levenson-Falk, USC
Abstract: Nonequilibrium populations of quasiparticle excitations plague superconducting circuits, causing errors in quantum processors. I will present our results using aluminum nanobridge Josephson junctions to detect quasiparticles as they trap and untrap in the junctions' internal Andreev bound states. By carefully studying the trapping and untrapping rates as a function of environmental conditions, we can find the quasiparticles' mechanisms of excitation, relaxation, and thermalization. I will further discuss novel filter designs that can eliminate the high-frequency radiation that generates the quasiparticles in the first place.
Host: Maxim Vavilov
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Plasma Seminar
Understanding the plasma universe through laboratory experiments and related models
Time: 1:00 pm - 2:00 pm
Place: B343 Sterling Hall
Speaker: Yang Zhang, Caltech
Abstract: Laboratory experiments and the models they inspire are powerful tools for studying the plasma universe. In this talk, I will present possible solutions to two big problems in the plasma universe, namely how solar flares are generated and how accretion disks transport angular momentum and generate astrophysical jets. Addressing the first problem, I will present observations from a laboratory experiment that simulates solar coronal loop physics. Transient, localized 7.6-keV X-ray bursts and a several-kilovolt voltage spike are observed to be associated with the breaking of braided magnetic flux ropes containing 2 eV plasma. These spikes occur when the braid strand radius is choked down to be at the kinetic scale by either MHD kink or magnetic Rayleigh–Taylor instabilities. The observed sequence reveals an MHD to non-MHD cross-scale coupling that is likely responsible for generating solar energetic particles and X-ray bursts. Addressing the second problem, I will present a first-principles angular momentum transport mechanism based only on collisions between neutrals and charged particles in the presence of gravitational and magnetic fields. I find that ions and electrons drift in opposite radial directions as a result of colliding with Kepler-motion neutrals. This reduces the ordinary angular momentum of neutrals and increases the canonical angular momentum of charged particles in a manner such that the net global canonical angular momentum is conserved. This process provides a gravitational dynamo converting gravitational energy into the electric energy that powers an astrophysical jet. The model predicts an accretion rate of 3 × 10−8 solar mass per year in good agreement with observed accretion rates. Finally, I will discuss my future research plans for using laboratory experiments and related models to study the plasma cosmos.
Host: Jan Egedal
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Astronomy Colloquium
Supercomputer Simulations of the Universe: Interfacing galaxy formation with precision cosmology
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Volker Springel, Max-Planck Institute for Astrophysics
Abstract: Numerical simulations of cosmic structure formation have become a powerful tool in astrophysics. Starting right after the Big Bang, they predict the dark matter backbone of the cosmic web far into the non-linear regime and follow complex galaxy formation physics with rapidly improving fidelity. In my talk, I will review the methodology and selected results of recent hydrodynamical galaxy formation simulations, such as the IllustrisTNG MillenniumTNG projects. In particular I will discuss some of the primary challenges in modelling strong, scale-dependent feedback processes that regulate star formation, and highlight the important role played by supermassive black holes in galaxy formation. I will also discuss extremely large simulations and describe how they help to make reliable predictions for the impact of baryons and massive neutrinos on cosmological observables, effects that need to be understood to make full use of upcoming new survey data.
Host: Ke Zhang
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