Dina Genkina

Visualizing edge states with an atomic Bose gas in the quantum Hall regime

Visualizing edge states in atomic systems Visualizing edge states in atomic systems Simulating the solid state using ultracold atoms is an appealing research approach. In solids, however, the charged electrons are susceptible to an external magnetic field, which curves their trajectories and makes them skip along the edge of the sample. To observe this phenomenon with cold atoms requires an artificial magnetic field to have a similar effect on the neutral atoms (see the Perspective by Celi and Tarruell). Stuhl et al. obtained skipping orbits with bosonic atoms using a lattice that consisted of an array of atoms in one direction and three internal atomic spin states in the other. In a complementary experiment, Mancini et al. observed similar physics with fermionic atoms. Science, this issue pp. 1514 and 1510; see also p. 1450 Skipping orbits of neutral bosonic rubidium-87 atoms are imaged after an artificial magnetic field is induced in a synthetic lattice. [Also see Perspective by Celi and Tarruell] Bringing ultracold atomic gases into the quantum Hall regime is challenging. We engineered an effective magnetic field in a two-dimensional lattice with an elongated-strip geometry, consisting of the sites of an optical lattice in the long direction and of three internal atomic spin states in the short direction. We imaged the localized states of atomic Bose-Einstein condensates in this strip; via excitation dynamics, we further observed both the skipping orbits of excited atoms traveling down the system’s edges, analogous to edge magnetoplasmons in two-dimensional electron systems, and a dynamical Hall effect for bulk excitations. Our technique involves minimal heating, which will be important for spectroscopic measurements of the Hofstadter butterfly and realizations of Laughlin’s charge pump.

Feshbach enhanced s-wave scattering of fermions: direct observation with optimized absorption imaging

We directly measured the normalized s-wave scattering cross-section of ultracold 40K atoms across a magnetic-field Feshbach resonance by colliding pairs of degenerate Fermi gases (DFGs) and imaging the scattered atoms. We extracted the scattered fraction for a range of bias magnetic fields, and measured the resonance location to be B0 = 20.206(15) mT with width Δ = 1.0(5) mT. To optimize the signal-to-noise ratio of atom number in scattering images, we developed techniques to interpret absorption images in a regime where recoil induced detuning corrections are significant. These imaging techniques are generally applicable to experiments with lighter alkalis that would benefit from maximizing signal-to-noise ratio on atom number counting at the expense of spatial imaging resolution.

Brownian motion of solitons in a Bose-Einstein condensate

Significance Solitons, spatially localized, mobile excitations resulting from an interplay between nonlinearity and dispersion, are ubiquitous in physical systems from water channels and oceans to optical fibers and Bose–Einstein condensates (BECs). From our pulse throbbing at our wrists to rapidly moving tsunamis, solitons appear naturally at a wide range of scales. In nonlinear optical fibers, solitons can travel long distances with applications for communication technology and potential for use in quantum switches and logic. Understanding how random processes contribute to the decay and the diffusion of solitons is essential to advancing these technologies. We observed and controlled the Brownian motion of solitons. We launched solitonic excitations in highly elongated Rb87 Bose–Einstein condensates (BECs) and showed that a dilute background of impurity atoms in a different internal state dramatically affects the soliton. With no impurities and in one dimension (1D), these solitons would have an infinite lifetime, a consequence of integrability. In our experiment, the added impurities scatter off the much larger soliton, contributing to its Brownian motion and decreasing its lifetime. We describe the soliton’s diffusive behavior using a quasi-1D scattering theory of impurity atoms interacting with a soliton, giving diffusion coefficients consistent with experiment.