Zhendong Zhang’s PhD Thesis Defense

Emergent collective phenomenon can happen when a BEC of dilute atomic gas is driven far from equilibrium. We found matter-wave jets emitted collectively out of a quasi-2D cesium condensate when the s-wave scattering length is periodically modulated. Even though locally the population in the jets follows thermal distribution, globally the system remains a pure state. We confirm both the spatial and temporal coherence from the interference fringes of jets with different momentum and time reversal of the jet emission, respectively. Before the jets leave condensate, these finite momentum modes interfere with the uniform BEC and form density waves. By modulating the interaction at two commensurate frequencies, momentum modes whose propagating directions separate by 60^o,90^o and 180^o are correlated. Patterns with D₆, D₄ and D₂ symmetries emerge under an unsupervised machine learning algorithm, which agrees with the result of conventional correlation functions. In particular, we found the D₆ pattern originates from an underlying resonant nonlinear wave mixing process, which leads to faster than exponential growth of the pattern strength. Moreover, we demonstrate that this interaction modulation technique can be used to do phase tomography of nonuniform and rotating condensates. In the second part of my talk, I will show you our recent experiment of making a BEC of g-wave molecules by pairing atoms in an atomic condensate near a narrow Feshbach resonance. The condensation of molecules is confirmed by equation of state measurement, from which we extract the molecular scattering length to be +220 Bohr. The two-dimensional and flat-bottomed trap geometry and low temperature help to stabilize the molecules and remain thermal equilibrium. Our work demonstrates the long-sought transition between atomic and molecular condensates, the bosonic analogue of BEC-BCS (Bardeen-Cooper-Schrieffer) crossover in a degenerate Fermi gas. Our recent investigation into the coherent chemical reaction dynamics between atoms and molecules in quantum degeneracy regime will also be presented.