# Browsing by Author "Thomas, John E"

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Item Open Access Optical Control of Magnetic Feshbach Resonances by Closed-Channel Electromagnetically Induced Transparency(2016) Jagannathan, ArunkumarOptical control of interactions in ultracold gases opens new fields of research by creating ``designer" interactions with high spatial and temporal resolution. However, previous optical methods using single optical fields generally suffer from atom loss due to spontaneous scattering. This thesis reports new optical methods, employing two optical fields to control interactions in ultracold gases, while suppressing spontaneous scattering by quantum interference. In this dissertation, I will discuss the experimental demonstration of two optical field methods to control narrow and broad magnetic Feshbach resonances in an ultracold gas of $^6$Li atoms. The narrow Feshbach resonance is shifted by $30$ times its width and atom loss suppressed by destructive quantum interference. Near the broad Feshbach resonance, the spontaneous lifetime of the atoms is increased from $0.5$ ms for single field methods to $400$ ms using our two optical field method. Furthermore, I report on a new theoretical model, the continuum-dressed state model, that calculates the optically induced scattering phase shift for both the broad and narrow Feshbach resonances by treating them in a unified manner. The continuum-dressed state model fits the experimental data both in shape and magnitude using only one free parameter. Using the continuum-dressed state model, I illustrate the advantages of our two optical field method over single-field optical methods.

Item Open Access Precision Measurement of the Sound Velocity in an Ultracold Fermi Gas Through the BEC-BCS Crossover(2010) Joseph, James AdlaiA trapped Fermi gas near a collisional resonance provides a unique laboratory for testing many-body theories in a variety of fields. The ultracold Fermi gas produced in our lab is comprised of the lowest two spin states of $^6$Li. At 834 G there is a collisional or Feshbach resonance between the two spin states. The scattering length between trapped atoms of opposing spins far exceeds the interparticle spacing of the gas. On resonance, a strongly interacting, unitary, Fermi gas is created which exhibits universal behavior. The unitary Fermi gas is a prototype for other exotic systems in nature from nuclear matter to neutron stars and high temperature superconductors.

For magnetic fields less than 834 G the scattering length is positive, and pairs Fermi atoms can form molecular dimers. These dimers, comprised of two fermions, are bosons. At ultracold temperatures the molecular bosons populate the lowest energy level and form a Bose Einstein Condensate (BEC). For magnetic fields greater than 834G the scattering length between fermions in opposing spin states is negative, like Cooper pairs formed between electrons in a superconductor. The Bardeen, Cooper, and Shriefer (BCS) theory was developed to describe the pairing effect in the context of superconductors. In our experiment we produce an ultracold unitary gas. By tuning the magnetic field to either side of the Feshbach resonance we can transform the gas into a weakly interacting BEC or BCS superfluid. Therefore, the region near a Feshbach resonance is called the BEC-BCS crossover.

This dissertation presents a precision measurement of the hydrodynamic sound velocity in an ultracold Fermi gas near a Feshbach resonance. The sound velocity is measured at various magnetic fields both above and below resonance. Moreover, we are able compare our measurements to theoretical descriptions of hydrodynamic sound propagation. Further, our measurement of sound velocity exactly reproduces the non-perturbative case, eliminating the need to consider nonlinear effects. At resonance the sound velocity exhibits universal scaling with the Fermi velocity to within 1.8\% over a factor of 30 in density. In a near zero temperature unitary gas the average sound velocity at the axial center was measured, $c(0)/v_F$ = 0.364(0.005), as well as the universal constant, $\beta$ = -0.565(0.015). The measurement of sound velocity in an ultracold gas throughout the BEC-BCS crossover provides further evidence of the continuous connection between the physics of the BEC, unitary, and BCS systems.

Item Open Access Quantum Transport and Scale Invariance in Expanding Fermi Gases(2014) Elliott, EthanThis dissertation describes the first experimental measurement of the energy and interaction dependent shear viscosity $\eta$ and bulk viscosity $\zeta$ in the hydrodynamic expansion of a two-component Fermi gas near a broad collisional (Feshbach) resonance. This expansion also provides a precise test of scale invariance and an examination of local thermal equilibrium as a function of interaction strength. After release from an anisotropic optical trap, we observe that a resonantly interacting gas obeys scale-invariant hydrodynamics, where the mean square cloud size $\langle{\mathbf{r}}^2\rangle=\langle x^2+y^2+z^2\rangle$ expands ballistically (like a noninteracting gas) and the energy-averaged bulk viscosity is consistent with zero, $0.00(0.04)\,\hbar\,n$, with $n$ the density. In contrast, the aspect ratios of the cloud exhibit anisotropic ``elliptic" flow with an energy-dependent shear viscosity. Tuning away from resonance, we observe conformal symmetry breaking, where $\langle{\mathbf{r}}^2\rangle$ deviates from ballistic flow. We find that $\eta$ has both a quadratic and a linear dependence on the interaction strength $1/({k_{FI}a})$, where $a$ is the s-wave scattering length and $k_{FI}$ is the Fermi wave vector for an ideal gas at the trap center. At low energy, the minimum is less than the resonant value and is significantly shifted toward the BEC side of resonance, to $1/(k_{FI}a) = 0.2$.

Item Open Access Radio Frequency Spectroscopy Of a Quasi-Two-Dimensional Fermi Gas(2013) Zhang, YingyiThis dissertation presents the first experiments on radio frequency (rf) spectroscopy of a quasi-two dimensional strongly interacting ultracold atomic Fermi gas. A 50-50 mixture of spin-up and spin-down atoms is confined in a series of pancake-shaped traps produced using an optical standing-wave. To make the system quasi-two dimensional, I adjust the Fermi energy in the weakly confined direction to be comparable to the harmonic oscillator energy level spacing in the tightly confined direction.

For a perfectly two dimensional system, at low enough temperature, spin-up and spin-down atoms should form dimers in the ground state of the tightly confined direction. However, in our quasi-two dimensional system I find that the simple dimer theory does not agree with the measured radio-frequency spectra. Instead, the data can be explained by polaron to polaron transitions, which is a many-body effect. Here, a polaron is a spin-down impurity surrounded by a cloud of particle-hole pairs in a spin-up Fermi sea. With this unique strongly interacting quasi-two dimensional system, I am able to study the interplay between confinement induced two-body pairing and many-body physics in confined mesoscopic systems of several hundred atoms, which has not been previously explored and offers new challenges for predictions.

Item Open Access Spin Imbalanced Quasi-Two-Dimensional Fermi Gases(2015) Ong, Willie Chuin HongSpin-imbalanced Fermi gases serve as a testbed for fundamental notions and are efficient table-top emulators of a variety of quantum matter ranging from neutron stars, the quark-gluon plasma, to high critical temperature superconductors. A macroscopic quantum phenomenon which occurs in spin-imbalanced Fermi gases is that of phase separation; in three dimensions, a spin-balanced, fully-paired superfluid core is surrounded by an imbalanced normal-fluid shell, followed by a fully polarized shell. In one-dimension, the behavior is reversed; a balanced phase appears outside a spin-imbalanced core. This thesis details the first density profile measurements and studies on spin-imbalanced quasi-2D Fermi gases, accomplished with high-resolution, rapid sequential spin-imaging. The measured cloud radii and central densities are in disagreement with mean-field Bardeen-Cooper-Schrieffer theory for a 2D system. Data for normal-fluid mixtures are well fit by a simple 2D polaron model of the free energy. Not predicted by the model is an observed phase transition to a spin-balanced central core above a critical polarization.

Item Open Access Ultracold Fermi Gases in a Bichromatic Optical Superlattice(2016) Cheng, ChingyunI describe the theory and construction of a new bichromatic optical superlattice to study the pairing and thermodynamics of spin $\frac{1}{2}$-up and spin $\frac{1}{2}$-down atoms in periodic double well potentials. Our bichromatic lattice contains $\lambda_1=1064$ nm and $\lambda_2=532$ nm standing wave lattices. With tunable depth and relative phase between the two lattices, periodic double well potentials of arbitrary local symmetry can be constructed.

I present the first systematic experimental study of a two-component ultracold $^6$Li atomic Fermi gas in a single color 1064 nm lattice, which is continuously tuned from 2D to quasi-2D. A system is 2D if it is free to move in two dimensions while tightly confined in the third direction, such that only the ground state is occupied. Conversely, it is quasi-2D if higher states in the tightly confined direction are also occupied. I describe both radio frequency spectra and radial cloud profiles measured under identical conditions for each regime. Our results confirm predictions that the mean-field theory is not valid throughout the 2D to quasi-2D dimensional crossover. We also clarify that there is no transition between 2D and quasi-2D systems.

I also present the first study of pairing in a periodic double well potential. A Green's function method is developed to compute the pairing energies in the lattice. Although further understanding of the results are needed, I provide some preliminary rf spectra measurements supporting the theoretical approach and implying the existence of two types of pairing.

Item Open Access Universal quantum viscosity in a unitary Fermi gas.(2012) Cao, ChenglinUnitary Fermi gases, first observed by our group in 2002, have been widely studied as they provide model systems for tabletop research on a variety of strongly coupled systems, including the high temperature superconductors, quark-gluon plasmas and neutron stars. A two component6Li unitary Fermi gas is created through a collisional Feshbach resonance centered near 834G, using all-optical trapping and cooling methods. In the vicinity of the Feshbach resonance, the atoms are strongly interacting and exhibit universal behaviors, where the equilibrium thermodynamic properties and transport coefficients are universal functions of the density n and temperature T. Thus, unitary Fermi gases provide a paradigm to study nonperturbative many-body physics, which is of fundamental significance and crosses several fields.This dissertation reports the first measurement of the quantum shear viscosity in a6Li unitary Fermi gas, which is also the first measurement of a transport coefficient for a unitary Fermi gas. While equilibrium thermodynamic quantities have been theoretically and experimentally studied for the past few year, the measurement of a transport coefficient for a unitary Fermi gas provides new challenges for state of the art nonperturbative many-body theory as transport coefficients are more difficult to calculate than equilibrium thermodynamic quantities. Two hydrodynamic experiments are employed to measure the shear viscosityηin different temperature regimes: an isotropic expansion is used for the high temperature regime and radial breathing mode is employed for the low temperature regime. In order to consistently and quantitatively extract the shear viscosity from these two experiments, hydrodynamic theory is utilized to derive universal hydrodynamic equations, which include both the friction force and the heating arising from viscosity. These equations are simplified and solved by considering the universal properties of unitary Fermi gases as well as the specific conditions for each experiment. Using these universal hydrodynamic equations, shear viscosity is extracted from the an isotropic expansion conducted at high temperatures and the predicted η ∝ T3/2 universal scaling is demonstrated. The demonstration of the high temperature scaling sets a benchmark for measuring viscosity at low temperatures. For the low temperature breathing mode experiment, the shear viscosity is directly related to the damping rate of an oscillating cloud, using the same universal hydrodynamic equations. The raw data from the previously measured radial breathing experiments are carefully analyzed to extract the shear viscosity. The low temperature data join with the high temperature data smoothly, which yields the full measurement of the quantum shear viscosity from nearly the ground state to the two-body Boltzmann regime.The possible effects of the bulk viscosity in the high temperature an isotropic expansion experiment is also studied and found to be consistent with the predicted vanishing bulk viscosity in the normal fluid phase at unitarity. Using the measured shear viscosityηand the previously measured entropy densitys, the ratio of η/s is estimated and compared to a string theory conjecture, which suggests that η/s≥~/4πkB for a broad class of strongly interacting quantum fluids and defines a perfect fluid when the equality is satisfied. It is found that η/s is about 5 times the string theory limit, for a unitary Fermi gas at the normal-superfluid transition point. This shows that our unitary Fermi gas exhibit nearly perfect fluidity at low temperatures. As presented part of this dissertation is the development of consistent and accurate methods of calibrating the energy and temperature for unitary Fermi gases. While the energy is calculated from the cloud dimensions by exploiting the virial theorem, the temperature is determined using different methods for different temperature regimes. At high temperatures, a universal second virial coefficient approximation is applied to the energy density, from which a variety of thermodynamic quantities, including the temperature, are derived in terms of the measured cloud size. For low temperatures, the previous calibration from the energy E and entropy S measurement is improved by using a better calculation of the entropy and adding constraints at high temperatures, using the second virial approximation. A power law curve with a discontinuous heat capacity is then fitted to the E-Scurve and the temperature is obtained using ∂ E/∂S. The energy and temperature calibrations developed in this dissertation are universal and therefore can be applied to other thermodynamic and hydrodynamic experiments at unitarity.