Researchers from Rice University were surprised when atoms disappeared during experiments involving ultra-cold temperatures after Bose Einstein condensates (BECs) prepared in solitons were collided.

In a study published in Nature Physics, the researchers showed that certain collisions had solitons approaching each other but maintaining a gap between them before bouncing away. This is surprising because researchers instead expected that colliding solitons would be passing through one another without changing shape or slowing down.

"You never see them together. There is always a hole, a gap that they must jump over. They pass through one another, but they never occupy the same space while they're doing that," said Randy Hulet, a Fayez Sarofim Professor of Physics and Astronomy at Rice.

According to him, the phenomenon is explained by wave packet interference. Suppose one soliton has a positive charge and the other negative. As the pair cancels each other out, it is unlikely that they would meet in one spot. Instead, the solitons would pass through the spot, although they'll never be seen actually occupying the spot as they pass.

To produce solitons, researchers balanced out the forces of repulsion and attraction in the BECs. BECs are thousands of lithium ions in clumps cooled to a millionth of a degree above absolute zero.

The researchers had cameras to capture the movement of the BECs. In the images taken, two solitons are showed oscillating back and forth in opposite directions. Thousands of collisions were documented for the experiment, showing indeed that pairs of solitons maintain a certain gap between them although that doesn't always happen.

To further explore the phenomenon they observed, the researchers need to conduct new experiments that focus on the defining feature of solitons that couldn't be controlled -- their phase.

Researchers tagged solitons to let them observe more closely by making one part of the pair larger than the other. In the new experiments, they were able to capture images of collisions between solitons that differed in size.

This new round of experiments confirmed what the researchers had previously expected: solitons pass through each other, only appearing to be bouncing away.

The study was led by Jason Nguyen, a postdoctoral research associate. Aside from Hulet and Nguyen, the research team also included: Paul Dyke, a former postdoctoral researcher from Rice; De Luo, a graduate student from Rice; and Boris Malomed, a faculty member from the department of physical electronics at Tel Aviv University.