Showing papers by "Cheng Chin published in 2015"

Quantum Dynamics with Spatiotemporal Control of Interactions in a Stable Bose-Einstein Condensate.

Abstract: Optical control of atomic interactions in quantum gases is a long-sought goal of cold atom research. Previous experiments have been hindered by rapid decay of the quantum gas and parasitic deformation of the trap potential. We develop and implement a generic scheme for optical control of Feshbach resonances which yields long quantum gas lifetimes and a negligible parasitic dipole force. We show that fast and local control of interactions leads to intriguing quantum dynamics in new regimes, highlighted by the formation of van der Waals molecules and localized collapse of a Bose condensate.

Roton-maxon excitation spectrum of Bose condensates in a shaken optical lattice.

Abstract: We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice modulation amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate---both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified Bogoliubov model.

Universal Loss Dynamics in a Unitary Bose Gas

Abstract: The low temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here we first present a theoretical model that describes the dynamic competition between two-body evaporation and three-body re-combination in a harmonically trapped unitary atomic gas above the condensation temperature. We identify a universal magic trap depth where, within some parameter range, evaporative cooling is balanced by recombination heating and the gas temperature stays constant. Our model is developed for the usual three-dimensional evaporation regime as well as the 2D evaporation case. Experiments performed with unitary 133 Cs and 7 Li atoms fully support our predictions and enable quantitative measurements of the 3-body recombination rate in the low temperature domain. In particular, we measure for the first time the Efimov inelasticity parameter $\eta$ * = 0.098(7) for the 47.8-G d-wave Feshbach resonance in 133 Cs. Combined 133 Cs and 7 Li experimental data allow investigations of loss dynamics over two orders of magnitude in temperature and four orders of magnitude in three-body loss. We confirm the 1/T 2 temperature universality law up to the constant $\eta$ *.

In Situ Imaging of Atomic Quantum Gases

Abstract: One exciting progress in recent cold atom experiments is the development of high resolution, in situ imaging techniques for atomic quantum gases [1-3]. These new powerful tools provide detailed information on the distribution of atoms in a trap with resolution approaching the level of single atom and even single lattice site, and complement the well developed time-of-flight method that probes the system in momentum space. In a condensed matter analogy, this technique is equivalent to locating electrons of a material in a snap shot. In situ imaging has offered a new powerful tool to study atomic gases and inspired many new research directions and ideas. In this chapter, we will describe the experimental setup of in situ absorption imaging, observables that can be extracted from the images, and new physics that can be explored with this technique.

Ten years of Nature Physics: Bound to be universal?

Abstract: Three papers published in Nature Physics in 2009 revealed the intriguing three- and four-body bound states arising from the predictions by Vitaly Efimov nearly half a century ago. But some of these findings continue to puzzle the few-body physics community.

Stable levitation and dynamics of ice particles at low pressures

Abstract: We demonstrate stable levitation and trapping of ice particles of 30~200 micon at low background gas pressures in the presence of a temperature gradient. The thermophoretic force levitates the particles, which have long lifetimes of over an hour. The equilibrium position depends on the background pressure and temperature gradient, which is consistent with theoretical expectations. Furthermore, we investigate interesting launching and merging dynamics of the levitated particles, as well as the development of instability at high background pressures. Our system provides a robust platform to investigate the aggregation of floating ice particles in air, and potentially chemical and biological processes in a microgravity environment.

Stable thermophoretic trapping of generic particles at low pressures

Abstract: We demonstrate levitation and three-dimensionally stable trapping of a wide variety of particles in a vacuum chamber through the use of the thermophoretic force in the presence of a strong temperature gradient. Typical sizes of the trapped particles are between 10 microns and 1 mm at a pressure between 1 and 10 Torr. The trapping stability is provided by the geometry of the temperature field, as well as the transition between the free molecule and hydrodynamic regimes of the thermophoretic force. To quantitatively measure the thermophoretic force, we examine the levitation heights of spherical polyethylene spheres under various experimental conditions and determine the temperature gradient needed to levitate the particles. A good agreement between our experimental observations and theoretical calculations is obtained. Our system offers a new platform to study thermophoretic phenomena and to simulate dynamics of interacting many-body systems in a microgravity environment.