Ph.D Thesis


Ph.D StudentShkedrov Constantine
SubjectHigh-Sensitivity rf and Raman Spectroscopy of a Quantum
Degenerate Fermi Gas
DepartmentDepartment of Physics
Supervisor ASSOCIATE PROF. Yoav Sagi
Full Thesis textFull thesis text - English Version


Abstract

This thesis describes the construction of a new experimental apparatus producing a degenerate Fermi gas of 40K and the subsequent experiments on this setup. My research was focused on: 1. Development of high-sensitivity radio frequency (rf) and Raman spectroscopy. 2. Generation and characterization of a homogeneous Fermi gas. 3. Combing these aforementioned techniques in a set of experiments that explore universality in the BEC-BCS crossover regime. As I was the first student in the group, I spent a considerable part of my Ph.D. building the new experiment, hence this thesis includes a detailed description of the experimental machine.


A new, highly sensitive spectroscopic method was developed for studying the rf spectrum of the strongly-interacting degenerate Fermi gas. Using this new method, a universal property of fermions with short-range interaction, namely a power-law scaling of the spectrum at high frequencies, was observed all the way up to the finite microscopic interaction range. In addition, a trap-averaged contact parameter was extracted from this measurement in the strongly-interacting regime. The high sensitivity spectroscopy allowed us to measure accurately the binding energy of these Feshbach molecules in an extended frequency range. We used this data to extract a better calibration of the Feshbach resonance parameters for the two lowest sublevels of 40K.


The experiments mentioned above were performed in a harmonic trap, in which the non-uniform atomic density complicates the interpretation of the results, broadens observables, and may potentially obscure phases that exist only a small region of the phase space. I constructed a trap which is mostly dark, has sharp repulsive walls, and levitates the atoms against gravity, to produce a nearly uniform gas. For simultaneous magnetic levitation of two spin states with different magnetic dipole moments, I developed a novel method which uses rf pulses that induces rapid Rabi oscillations to average out the residual potential difference. Using absorption imaging, I demonstrated that this procedure yields a homogeneous density distribution for both spins and does not lead to heating. I also showed theoretically and experimentally that the mixing rf pulse does not introduce coherence and does not modify the many-body state.


To measure the temperature of the gas in the new trapping potential, we developed a new scheme instead of the standard time-of-flight measurement, which is inaccurate due to the large initial size and ultra-low temperature of the gas. Based on velocity-selective Raman transitions, our novel spectroscopic measurement yields the in-trap one-dimensional momentum distribution of the gas. We tested and verified this method with harmonically trapped gas, where the conventional time-of-flight is applicable. Then, we applied it to the gas in the uniform trap and showed that its momentum distribution is consistent with that of a uniform gas and extracted the temperature from it.


Lastly, I applied the high-sensitivity rf spectroscopy to the uniform gas, and studied the normal state in the BEC-BCS regime. The same universal scaling is observed also for the homogeneous gas. The homogeneous contact parameter and the mean-field energy shift were extracted from these measurements.