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Events on Tuesday, April 27th, 2021

Thesis Defense
Heterostructure Modifications, Fabrication Improvements, and Measurement Automation of Si/SiGe Quantum Dots for Quantum Computation
Time: 9:00 am
Place: https://uwmadison.zoom.us/j/96050486034?pwd=VlU2VHp3NmVSdkhMalNuRGw5eFZ4QT09
Speaker: Thomas McJunkin, Physics PhD Graduate Student
Abstract: Quantum computing -- leveraging quantum phenomena to perform complex and intractable computational problems -- has rapidly progressed from a theoretical aspiration to a near reality. Currently, there are many competing approaches to the way the physical qubits (quantum bits) are built, from trapped ions, to superconducting circuits, to semiconductor quantum dots, and beyond. Here, we focus on quantum dots, where electrons or holes are confined within a semiconductor and the quantized nature of charge and spin are utilized for computation. Within the field of quantum dots, heterostructures made of silicon and silicon germanium are especially enticing due to their low density of defects and nuclear spin. Although quantum dots are a promising avenue for quantum computation because of their intrinsically small size and similarity to classical transistors, nearly every aspect of their design, realization, and control has yet to be fully optimized. This thesis explores modifications to the heterostructure, fabrication, and measurement of Si/SiGe quantum dots in the pursuit of improved quantum dot qubits. The valley splitting in silicon quantum dots, a near degeneracy of the lowest lying energy states, is critical to the formation and performance of silicon qubits. In this work, we present several modifications to the Si/SiGe heterostructure in an effort to enhance this splitting. In particular, we investigate the effects of introducing germanium to the silicon quantum well by the inclusion of a single spike in germanium concentration or an oscillatory concentration throughout the well. We present experimental measurements of the energy spectrum in both modifications and, coupled with theoretical support, claim enhancements to the valley splitting. Next, we present several fabrication techniques with the goal of improved quantum dot functionality and lowered charge noise, a major barrier to higher quality devices. We report a new strategy for etched palladium fabrication and discuss the current progress. Finally, we present work towards the automation of quantum dot tuning. As quantum dot devices increase in the number of qubits, so do the number of electrostatic gates which control the device. We discuss the development of automated tuning procedures and present a routine for the formation of well-controlled quantum dots from the initial voltage settings. Join Zoom Meeting https://uwmadison.zoom.us/j/96050486034?pwd=VlU2VHp3NmVSdkhMalNuRGw5eFZ4QT09 Meeting ID: 960 5048 6034 Passcode: qubits
Host: Mark Eriksson, Faculty Advisor
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Institute for Nuclear Energy Systems (INES) Colloquium
Multiphysics analysis of fusion nuclear systems with the MOOSE framework
Time: 12:00 pm
Place: Zoom Meeting
Speaker: Dr. Paul Humrickhouse, leads the Fusion Safety Program at Idaho National Laboratory
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Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
Axion Production in Magnetars
Time: 2:00 pm
Place: https://berkeley.zoom.us/j/91922781599
Speaker: Steven Harris, INT-U. Washington
Abstract: The dense core of a neutron star may provide an ideal environment for the production of axions. Axions created in the core via nucleon bremsstrahlung processes would escape the neutron star and, with some conversion efficiency, turn into photons in the magnetosphere. The large external magnetic field of a magnetar enhances the conversion probability, making magnetars a particularly strong source of axion-induced photons. I will review the mechanisms of axion production in dense nuclear matter and calculate the energy spectrum of the photons coming from axion-photon conversion. This spectrum is compared to observed hard X-ray spectra from several magnetars in the McGill magnetar catalog to put limits on the product of the axion couplings to nucleons and photons as a function of the magnetar core
Host: Baha Balantekin
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Thesis Defense
The Role of Chaos and Magnetic Fields in the Cosmic Ray Anisotropy
Time: 3:00 pm
Place: https://uwmadison.zoom.us/j/98775530388?pwd=MTd6V0VxRGZxUGNSQ1N0VzVmZnRXZz09
Speaker: Vanessa Lopez Barquero , Physics PhD Graduate Student
Abstract: Join Zoom Meeting https://uwmadison.zoom.us/j/98775530388?pwd=MTd6V0VxRGZxUGNSQ1N0VzVmZnRXZz09 Meeting ID: 987 7553 0388 Passcode: 434580
Host: Francis Halzen, Faculty Advisor
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