The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Wild, Robert Johannes (Ph.D., Physics) Contact Measurements on a Strongly Interacting Bose Gas Thesis directed by Dr. Eric A. Cornell A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is potentially useful given the decreasing stability of BECs with increasing interactions. Successfully measuring the contact requires careful control of experimental parameters such as the magnetic field and the RF probe pulse. In this thesis I report on measurements of the contact, using RF spectroscopy, for an 85Rb atomic Bose-Einstein condensate (BEC). The measured contact exhibits beyond-mean-field behavior, the degree of which is dependent on the rates of change
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Wild, Robert Johannes (Ph.D., Physics) Contact Measurements on a Strongly Interacting Bose Gas Thesis directed by Dr. Eric A. Cornell A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with delta-function interactions. For bosons, the fact that contact spectroscopy can be used to probe the gas on short timescales is potentially useful given the decreasing stability of BECs with increasing interactions. Successfully measuring the contact requires careful control of experimental parameters such as the magnetic field and the RF probe pulse. In this thesis I report on measurements of the contact, using RF spectroscopy, for an 85Rb atomic Bose-Einstein condensate (BEC). The measured contact exhibits beyond-mean-field behavior, the degree of which is dependent on the rates of change
A powerful set of universal relations, centered on a quantity called the contact, connects the strength of short-range two-body correlations to the thermodynamics of a many-body system with zero-range interactions. We report on measurements of the contact, using rf spectroscopy, for an (85)Rb atomic Bose-Einstein condensate (BEC). For bosons, the fact that contact spectroscopy can be used to probe the gas on short time scales is useful given the decreasing stability of BECs with increasing interactions. A complication is the added possibility, for bosons, of three-body interactions. In investigating this issue, we have located an Efimov resonance for (85)Rb atoms with loss measurements and thus determined the three-body interaction parameter. In our contact spectroscopy, in a region of observable beyond-mean-field effects, we find no measurable contribution from three-body physics.
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Stewart, John (Ph.D., Physics) Probing a strongly interacting Fermi gas Thesis directed by Professor Dr. Deborah S. Jin This thesis presents experimental work probing a strongly interacting Fermi gas of atoms. The work presented here contributed to the demonstration that strongly interacting Fermi gas systems are accessing universal physics. The uni-versality of these systems means that we can use an atomic Fermi gas to gain an understanding of strongly interacting Fermions occurring elsewhere in nature, such as nuclear and neutron matter, high transition temperature superconduc-tors, and the quark-gluon plasma. Once the universality of these systems was verified, experiments were performed to extract the microscopic quantities of an
A dilute Bose-Einstein condensate (BEC) near a Feshbach resonance provides experimental physics with a clean and controllable system to investigate strongly interacting many-body systems. The ability to tune the scattering length allows BECs to be projected onto strong interactions from an initial weakly interacting state. However, historically, studying a bulk 3D strongly interacting BEC has been diffcult, as these systems are inherently unstable due to three-body inelastic collisions. Thus, 2D, 1D, lattice confined, and two component Fermi gases were used to explore the strong interactions in an ultracold gas. In this thesis, I present the first measurement of a strongly interacting 3D 85Rb BEC. I introduce our experimental system and the techniques we used to probe the BEC. I first report on probes of a BEC in the weakly interacting regime. To probe a spherical 85Rb BEC with strong interactions we implemented a novel experimental technique that allowed us to quickly change the interactions, bypassing inelastic losses. We projected the BEC onto unitarity, where the scattering length diverges and the interactions are infinite, in order to observe dynamics and the unexpectedly long lifetime of the gas. Additionally, we observe a universality of the gas with respect to the length scale set by the interparticle spacing.
A dilute gas Bose-Einstein condensate (BEC) near a Feshbach resonance offers a system that can be tuned from the well-understood regime of weak interactions to the complex regime of strong interactions. Strong interactions play a central role in the phenomena of superfluidity in liquid He, and theoretical treatments for this regime have existed since the 1950’s. However, these theories have not been experimentally tested as superfluid He offers no similar mechanism with which to tune the interactions. In dilute gas condensates near a Feshbach resonance, where interactions can be tuned, strong interactions have proven difficult to study due to the condensate’s metastable nature with respect to the formation of weakly bound molecules. In this thesis, I introduce an experimental system and novel probes of the gas that have been specifically designed to study strongly interacting BECs. I present Bragg spectroscopy measurements that have accessed this regime, as well as proof-of-principle experiments using photon-counting for Bragg spectroscopy at low-momentum. Finally, I show preliminary data using contact spectroscopy, which is a method that could lead to the first measurements of the predicted interaction energies for a dilute Bose gas of atoms in the strongly interacting regime.
This thesis investigates the response of coherently coupled spin-mixture Bose-Einstein condensates across a broad range of nonlinear regimes, extending from the deep-Rabi regime to the strongly nonlinear regime. On the theoretical side, collision processes induced and enhanced by strong coherent coupling are analyzed within a perturbative framework. Interaction channels arising from both mean-field and beyond-mean-field processes are identified, providing a microscopic description of the mechanisms through which few-body interactions contribute to the energy of the many-body system. On the experimental side, precise control over the interaction landscape is achieved in a 39-K spin mixture through improved stabilization of the magnetic field near 57 G, the operating point of the system. A feedforward compensation circuit is implemented to suppress magnetic-field noise, and Ramsey spectroscopy measurements demonstrate a twelvefold reduction in field fluctuations, corresponding to a residual root-mean-square noise level of a few tens of μG. This enhanced stability enables controlled measurements of the density-dependent equation of state of the coherently driven two-component condensate. As the density is increased, the system exhibits distinct interaction regimes. At low densities, the equation of state is dominated by repulsive two-body interactions. At intermediate densities, three-body contributions emerge and become increasingly relevant. At higher densities, the interaction energy shows an unusual saturation-like behavior. This behaviour reflects the nontrivial interplay between coherent coupling and interactions.
This thesis investigates two topics: ultracold atomic collisions in a radio-frequency field and universal properties of a degenerate unitary Bose gas. One interesting point of the unitary Bose gas is that the system has only one length scale, that is, the average interparticle distance. This single parameter determines all properties of the gas, which is called the universality of the system. We first introduce a renormalized contact interaction to extend the validity of the zero-range interaction to large scattering lengths. Then this renormalized interaction is applied to many-body theories to determined those universal relations of the system. From the few-body perspective, we discuss the scattering between atoms in a single-color radio-frequency field. Our motivation is proposing the radio-frequency field as an effective tool to control interactions between cold atoms. Such a technique may be useful in future experiments such as creating phase transitions in spinor condensates. We also discuss the formation of ultracold molecules using radio-freqency fields from a time-dependent approach.