Graduate Program Events

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Events on Wednesday, June 14th, 2023

Thesis Defense
Characterization of noise sources in semiconductor qubit devices
Time: 9:00 am - 11:00 am
Speaker: Yujun Choi, Department of Physics Graduate Student
Abstract: Quantum computing has garnered substantial attention in recent decades for its potential applications across various domains such as cybersecurity, chemical engineering, logistics optimization, data search, drug synthesis, and machine learning. However, practical utilization of quantum computing faces significant challenges due to considerable overhead. Even with the aid of state-of-the-art quantum error correction codes, millions of physical quantum bits (qubits) are required. This necessity arises from limitations in gate fidelities of qubits resulting from environmental noise. Therefore, it is imperative to investigate noise source characteristics and devise strategies to mitigate their impact on qubits.

Semiconductor qubits offer a promising platform that can be readily expanded by leveraging existing semiconductor industry facilities. In semiconductor devices, an array of detrimental noise sources, such as charge noise, hyperfine noise, evanescent-wave Johnson noise, and phonon-induced noise, can degrade coherence of the qubits. This dissertation specifically focuses on charge noise (1/f noise).

The dissertation commences by introducing a methodology to characterize diverse noise sources through the measurement of coherence times while rotating a vector magnet in a spin qubit device. Subsequently, it presents the application of a technique called noise source driving to enhance coherence of qubits. This approach involves applying an oscillating electric field to the charge noise sources. Following this, the dissertation elucidates a plausible mechanism explaining the pulse-induced resonance frequency shift of a qubit with fluctuations of two-level systems. Lastly, the dissertation discusses future research directions and concludes with closing remarks
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Thesis Defense
Measurement of the Astrophysical Diffuse Neutrino Flux using Starting Track Events in IceCube
Time: 11:00 am - 1:00 pm
Place: 4274 Chamberlin and
Speaker: Manuel Silva, Department of Physics Graduate Student
Abstract: Since the discovery of cosmic rays by Victor Hess in 1912, there have been numerous advances in the field including but not limited to: larger cosmic ray detectors, observatories searching for gamma-rays and astrophysical neutrinos. This dissertation focuses on neutrinos since they are neutral and light making them the optimal messenger. By measuring the neutrino flux, we can better under the mechanisms by which these cosmic rays are created. In 2013, the IceCube collaboration first announced the observation of these astrophysical neutrinos launching us into the era of high energy neutrino astrophysics. This work summarizes a novel dataset, searching for starting track events, whereas a neutrino interacts within the fiducial volume of the detector producing a muon track. This event morphology is of particular importance as it enables us to measure the energy of the event to within 25% error and the direction of the event to within 1.5◦ error. Utilizing 10.3 years of IceCube data, we characterize the astrophysical diffuse neutrino flux. Using a single power law flux, we measure the spectral index as γ = 2.58+0.10−0.09 and the per-flavor normalization as ΦAstro per−flavor = 1.68+0.19 −0.22 (at 100 TeV). The sensitive energy range to this flux is 3-550 TeV making this the lowest energy astrophysical flux measurement to date. We also show structure (or lack thereof) in the flux towards lower energies by using a broken power law. We reject γ &lt; 1 to 3σ significance and γ &lt; 2 to 2.1σ significance below a break energy of 25 TeV.
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