collide with = keep the state with?

Context:

The BEC is then exposed to a state-dependent potential which is exerted by a second microwave field. "Under the influence of this field the atoms are only allowed to collide with atoms of the same spin state. Therefore the dynamic evolution of their states depends on the states of all other atoms. This effect leads to an entanglement of the atoms," explains Max Riedel, doctoral student at the experiment.

More:

http://www.sciencedaily.com/releases/2010/04/100401101810.htm