Fig. 6: Fluid-flow tracing in the presence of a repulsive or attractive potential.
a A repulsive potential with waist σ ≈ 12 μm is located h = 68.0(5) μm below the trapped BEC. A small horizontal stripe of atoms (located in the region between the trapped BEC and the potential) is converted back to the \(\left|1,-1\right\rangle\) state by a series of brief 100 μs mw pulses in 0.5 ms intervals, leaving a void of atoms in the imaged \(\left|2,-2\right\rangle\) state. Overlaying the experimental image are the corresponding classical trajectories at these fixed ti, shown as dotted (red) curves for positive initial impact parameters 0 < x0 < 100 μm. These correspond to slices through the embedded sheet (xi, ti) ↦ (x, z, ti) (see for example Fig. 1c), demonstrating that these fluid-flow tracers provide a direct way to visualize the folding of this sheet which gives rise to the caustics. b Same as a, but for an attractive potential with waist σ ≈ 25 μm located h = 50(5) μm below the trapped BEC. Both of the experimental images here have been averaged over 101 runs.
