Oscillating soliton/vortex ring
The matter-wave interference between two initially-separated Bose-Einstein condensates was a landmark observation, demonstrating phase-coherence. The collisional energy of the two condensates was sufficient that interactions between atoms could be neglected. But what happens when the two condensates are separated by a very small distance, on the order of one healing length? Interactions can no longer be neglected at such low collisional energies. We created such a situation using our high-resolution imaging. We observed that the matter wave interference pattern becomes a single pair of gray solitons.
The story does not end here. It is generally known that a gray soliton is unstable in a 3D condensate. It should decay into a vortex ring. We found a different behavior, however. In the elongated condensate, each of the two solitons evolves into a vortex ring, and then back into a soliton. This process repeats periodically. Such a periodic oscillation between two qualitatively different forms seems to be a rare phenomenon.
Shomroni, I., Lahoud, E., Levi, S. & Steinhauer, J. Evidence for an oscillating soliton/vortex ring by density engineering of a Bose-Einstein condensate. Nature Physics 5, 193 (2009).