Kevin Wright

Superflow in a Toroidal Bose-Einstein Condensate: An Atom Circuit with a Tunable Weak Link

We have created a long-lived (≈40 s) persistent current in a toroidal Bose-Einstein condensate held in an all-optical trap. A repulsive optical barrier across one side of the torus creates a tunable weak link in the condensate circuit, which can affect the current around the loop. Superflow stops abruptly at a barrier strength such that the local flow velocity at the barrier exceeds a critical velocity. The measured critical velocity is consistent with dissipation due to the creation of vortex-antivortex pairs. This system is the first realization of an elementary closed-loop atom circuit.

Driving phase slips in a superfluid atom circuit with a rotating weak link.

We have observed well-defined phase slips between quantized persistent current states around a toroidal atomic (23Na) Bose-Einstein condensate. These phase slips are induced by a weak link (a localized region of reduced superfluid density) rotated slowly around the ring. This is analogous to the behavior of a superconducting loop with a weak link in the presence of an external magnetic field. When the weak link is rotated more rapidly, well-defined phase slips no longer occur, and vortices enter into the bulk of the condensate. A noteworthy feature of this system is the ability to dynamically vary the current-phase relation of the weak link, a feature which is difficult to implement in superconducting or superfluid helium circuits.

Creation and detection of Skyrmions in a Bose-Einstein condensate.

We present the first experimental realization and characterization of two-dimensional Skyrmions and half-Skyrmions in a spin-2 Bose-Einstein condensate. The continuous rotation of the local spin of the Skyrmion through an angle of pi (and half-Skyrmion through an angle of pi/2) across the cloud is confirmed by the spatial distribution of the three spin states as parametrized by the bending angle of the l vector. The winding number w = (0,1,2) of the internal spin states comprising the Skyrmions is confirmed through matter-wave interference.

Sculpting the Vortex State of a Spinor BEC

We use Raman-detuned laser pulses to achieve spatially varying control of the amplitude and phase of the spinor order parameter of a Bose-Einstein condensate. We present experimental results confirming precise radial and azimuthal control of amplitude and phase during the creation of vortex-antivortex superposition states.

Partial-transfer absorption imaging: A versatile technique for optimal imaging of ultracold gases

Partial-transfer absorption imaging is a tool that enables optimal imaging of atomic clouds for a wide range of optical depths. In contrast to standard absorption imaging, the technique can be minimally destructive and can be used to obtain multiple successive images of the same sample. The technique involves transferring a small fraction of the sample from an initial internal atomic state to an auxiliary state and subsequently imaging that fraction absorptively on a cycling transition. The atoms remaining in the initial state are essentially unaffected. We demonstrate the technique, discuss its applicability, and compare its performance as a minimally destructive technique to that of phase-contrast imaging.

Raman coupling of Zeeman sublevels in an alkali-metal Bose-Einstein condensate

We investigate amplitude and phase control of the components of the spinor order parameter of a 87 Rb Bose-Einstein condensate. By modeling the interaction of the multilevel atomic system with a pair of Raman-detuned laser pulses, we show that it is possible to construct a pulse-sequence protocol for producing a desired state change within a single Zeeman manifold. We present several successful elementary tests of this protocol in both the F = 1 and F = 2 Zeeman manifolds of 87 Rb using the D1 transitions. We describe specific features of the interaction which are important for multimode, spatially-varying field configurations, including the role of state-dependent, light-induced potentials.

Probing the Circulation of Ring-shaped Bose-Einstein Condensates

Joint Quantum Institute, National Institute of Standards and Technologyand the University of Maryland, Gaithersburg, MD 20899, USA(Dated: January 10, 2014)This paper reports the results of a theoretical and experimental study of how the initial circulationof ring–shaped Bose–Einstein condensates (BECs) can be probed by time–of–flight (TOF) images.We have studied theoretically the dynamics of a BEC after release from a toroidal trap potential bysolving the 3D Gross–Pitaevskii (GP) equation. The trap and condensate characteristics matchedthose of a recent experiment. The circulation, experimentally imparted to the condensate by stirring,was simulated theoretically by imprinting a linear azimuthal phase on the initial condensate wavefunction. The theoretical TOF images were in good agreement with the experimental data. Wefind that upon release the dynamics of the ring–shaped condensate proceeds in two distinct phases.First, the condensate expands rapidly inward, filling in the initial hole until it reaches a minimumradius that depends on the initial circulation. In the second phase, the density at the inner radiusincreases to a maximum after which the hole radius begins slowly to expand. During this secondphase a series of concentric rings appears due to the interference of ingoing and outgoing matterwaves from the inner radius. The results of the GP equation predict that the hole area is a quadraticfunction of the initial circulation when the condensate is released directly from the trap in whichit was stirred and is a linear function of the circulation if the trap is relaxed before release. Thesescalings matched the data. Thus, hole size after TOF can be used as a reliable probe of initialcondensate circulation. This connection between circulation and hole size after TOF will facilitatefuture studies of atomtronic systems that are implemented in ultracold quantum gases.

Threshold for creating excitations in a stirred superfluid ring

We have measured the threshold for creating long-lived excitations when a toroidal Bose-Einstein condensate is stirred by a rotating (optical) barrier of variable height. When the barrier height is on the order of or greater than half of the chemical potential, the critical barrier velocity at which we observe a change in the circulation state is much less than the speed for sound to propagate around the ring. In this regime we primarily observe discrete jumps (phase slips) from the non-circulating initial state to a simple, well-defined, persistent current state. For lower barrier heights, the critical barrier velocity at which we observe a change in the circulation state is higher, and approaches the effective sound speed for vanishing barrier height. The response of the condensate in this small-barrier regime is more complex, with vortex cores appearing in the bulk of the condensate. We find that the variation of the excitation threshold with barrier height is in qualitative agreement with the predictions of an effective 1D hydrodynamic model.

Phase fluctuations in anisotropic Bose condensates: from cigars to rings

We study the phase-fluctuating condensate regime of ultracold atoms trapped in a ring-shaped trap geometry, which has been realized in recent experiments. We first consider a simplified box geometry, in which we identify the conditions to create a state that is dominated by thermal phase fluctuations, and then explore the experimental ring geometry. In both cases we demonstrate that the requirement for strong phase fluctuations can be expressed in terms of the total number of atoms and the geometric length scales of the trap only. For the ring-shaped trap we discuss the zero temperature limit in which a condensate is realized where the phase is fluctuating due to interactions and quantum fluctuations. We also address possible ways of detecting the phase-fluctuating regime in ring condensates.

Protocols for dynamically probing topological edge states and dimerization with fermionic atoms in optical potentials

Topological behavior has been observed in quantum systems including ultracold atoms. However, background harmonic traps for cold-atoms hinder direct detection of topological edge states arising at the boundary because the distortion fuses the edge states into the bulk. We propose experimentally feasible protocols to probe localized edge states and dimerization of ultracold fermions. By confining cold-atoms in a ring lattice and changing the boundary condition from periodic to open using an off-resonant laser sheet to cut open the ring, topological edge states can be generated. A lattice in a topological configuration can trap a single particle released at the edge as the system evolves in time. Alternatively, depleting an initially filled lattice away from the boundary reveals the occupied edge states. Signatures of dimerization in the presence of contact interactions can be found in selected correlations as the system boundary suddenly changes from periodic to open and exhibit memory effects of the initial state distinguishing different configurations or phases.