School of Physics Thesis Dissertation Defense

Presenter:          Feng Xiong

Title:                    Achieving a Quantum Simulator in Ultracold Fermionic Systems

Date:                   Friday, July 14th, 2023

Time:                   10:00 AM

Location:            Howey N110

Virtual:               https://gatech.zoom.us/j/99252704038?pwd=N25BdGxVMDRIWjIzek5NQzRTQndNUT09

Committee: 

Dr. Colin Parker, School of Physics, Georgia Institute of Technology (advisor)

Dr. Michael Chapman, School of Physics, Georgia Institute of Technology

Dr. Brian Kennedy, School of Physics, Georgia Institute of Technology

Dr. Martin Mourigal, School of Physics, Georgia Institute of Technology

Dr. Joshua Kretchmer, School of Chemistry and Biochemistry, Georgia Institute of Technology

Abstract:

Real world material systems often have properties with roots in quantum mechanics which we are interested in. Studying such systems by classical models is often unsuitable, being either ineffective or inefficient. The general approach is through quantum simulations, in which laser cooled and trapped atoms are used as simulators. This thesis presents our study of ultracold quantum gases of Li-6, signifying our progress in building a quantum simulator. At first, we demonstrate the achievement of molecular BECs of Li-6 in its lowest and second lowest hyperfine state pairs by an all-optical method. We employ mostly standard techniques, but also introduce several unique features in our hardware system. Then, by preparing a degenerate Fermi gas of Li-6 in a mixture of its second lowest two hyperfine states and measuring its spin susceptibility in the BEC-BCS crossover, we study the “pseudogap” effects and compare it to the high-Tc cuprates. We develop a novel radiofrequency method to map the mixture to an RF-dressed basis. Imbalances are created between thermally equilibrium RF-dressed states, from which the spin susceptibilities are extracted over the interaction strength-temperature phase diagram. The results of such measurements for gases in the strongly interacting regions are compared to a mean-field model, to the ideal Fermi gas model, and to experimental results from several other publications. Lastly, we implement a 1D optical lattice. We tune the single particle dispersion relation using a shaken lattice by Floquet engineering. The driving signal is modulated through an IQ modulator fed to two AOMs. By loading a molecular BEC of Li-6 pairs into the shaking lattice, we have achieved coupling between the first two energy bands resulting in a double-well dispersion. The major result of our observations is that the atomic cloud under the inverted dispersion bifurcats into two soliton-like peaks in the momentum space. While in the position space, a density corrugation is formed in the atom cloud, which is caused by the two bifurcated wave peaks with opposing momentum beginning to separate. We have not yet fully understood the mechanism behind this phenomenon. For now, we model the result semi-classically by the Gross-Pitaevskii equation. The numerical simulations match reasonably well with the experimental results.