Changhyun Ryu

Quantized Rotation of Atoms from Photons with Orbital Angular Momentum

We demonstrate the coherent transfer of the orbital angular momentum of a photon to an atom in quantized units of variant Plancks over 2pi, using a 2-photon stimulated Raman process with Laguerre-Gaussian beams to generate an atomic vortex state in a Bose-Einstein condensate of sodium atoms. We show that the process is coherent by creating superpositions of different vortex states, where the relative phase between the states is determined by the relative phases of the optical fields. Furthermore, we create vortices of charge 2 by transferring to each atom the orbital angular momentum of two photons.

Observation of a 2D Bose Gas: From Thermal to Quasicondensate to Superfluid

We present experimental results on a Bose gas in a quasi-2D geometry near the Berezinskii, Kosterlitz, and Thouless (BKT) transition temperature. By measuring the density profile after time of flight and the coherence length, we identify different states of the gas. We observe that the gas develops a bimodal distribution without long range order. In this regime, the gas presents a longer coherence length than the thermal cloud; it is quasicondensed but is not superfluid. Experimental evidence indicates that we also observe the superfluid transition (BKT transition). For a sufficiently long time of flight, we observe a trimodal distribution when the gas has developed a superfluid component.

Experimental realization of Josephson junctions for an atom SQUID.

We report the creation of a pair of Josephson junctions on a toroidal dilute gas Bose-Einstein condensate (BEC), a configuration that is the cold atom analog of the well-known dc superconducting quantum interference device (SQUID). We observe Josephson effects, measure the critical current of the junctions, and find dynamic behavior that is in good agreement with the simple Josephson equations for a tunnel junction with the ideal sinusoidal current-phase relation expected for the parameters of the experiment. The junctions and toroidal trap are created with the painted potential, a time-averaged optical dipole potential technique which will allow scaling to more complex BEC circuit geometries than the single atom-SQUID case reported here. Since rotation plays the same role in the atom SQUID as magnetic field does in the dc SQUID magnetometer, the device has potential as a compact rotation sensor.

High-order quantum resonances observed in a periodically-kicked Bose condensate

We have observed high-order quantum resonances in a realization of the quantum delta-kicked rotor, using Bose-condensed sodium atoms subjected to a pulsed standing wave of laser light.

Rotating a Bose-Einstein Condensate Using Photons with Orbital Angular Momentum

We demonstrate the coherent transfer of the orbital angular momentum of photons to atoms using a stimulated 2-photon Raman process with Laguerre-Gaussian beams. The process is used to create superpositions of rotational (vortex) atomic states.

Bragg spectroscopy of a Bose condensate in a 3D optical lattice

We have measured the excitation spectrum of an /sup 87/Rb Bose-Einstein condensate loaded in a three dimensional optical lattice using Bragg spectroscopy.