Vortex nucleation in a rotating optical lattice of ultracold atoms
Ross Williams
Abstract
Vortex nucleation in a rotating optical lattice of ultracold atoms Ross Williams, Brasenose College, Oxford D.Phil thesis, Michaelmas 2009 This thesis describes the design and implementation of a two-dimensional rotating optical lattice for ultracold atoms. Rotating optical lattices for ultracold atoms have generated a great deal of theoretical interest due to the close analogy between electrons under a magnetic field and rotating neutral atoms. However, such is the difficulty associated with creating rotating lattice potentials which do not cause significant heating of the atoms there has been only one previous experimental realisation of a practical device, [Phys. Rev. Lett. 97, 240402 (2006)]. There S. Tung et al. reported that heating due to rotation limited the optical lattice to depths of less than 30% of the condensate’s chemical potential. We describe a new method for generating a rotating optical lattice, based on acousto-optic deflection and forming the lattice in the focal plane of a high numerical aperture lens. The intrinsically smooth rotation and excellent degree of control over the rotation frequency enabled a new breed of experiments to be performed, allowing the weak-lattice regime to be surpassed for the first time. In particular we report on the first experiments with a rotating optical lattice where lattice depths were reached such that a two-dimensional array of weakly-linked condensates was created, realising a bosonic Josephson-junction array under an effective magnetic field. We observe a new method of vortex nucleation, whereby rotation-induced phase differences between neighbouring condensates are converted to vortices upon ramping down the lattice. The arrangement for generating a rotating optical lattice also allowed the first realisation of a two-dimensional optical lattice with variable periodicity, known as an accordion lattice. Calibration of the accordion lattice is described, together with a discussion on the crossover from quantum to classical dynamics in large periodicity optical lattices.