Direct visualization of strong atom--atom interactions with colliding BECs

Macroscopic quantum properties of matter can hardly become more tangible than in the 1997 experiment\footnote{M. R. Andrews, {\it et al.}, Science \textbf{275}, 637 (1997).} where an interference pattern was literally seen by imaging the collision of two BECs comprised of weakly interacting atoms. Extending such a study to strong interactions is more challenging, but feasible, following an experimental success\footnote{P. Makotyn, {\it et al.}, Nat. Phys. \textbf{10}, 116--119 (2014).} in rapidly quenching a BEC from weak to strong atom--atom interactions. A recently developed cluster-expansion approach\footnote{M.~Kira, Ann.~Phys.~{\bf 356}, 185--243 (2015).} yields a nonperturbative description of strongly interacting BECs, and it has been demonstrated to quantitatively explain\footnote{M. Kira, Nat. Commun. \textbf{6} 6624 (2015).} experiments. Here, we generalize this method to describe collision of two BECs and a simultaneous quench of atom--atom interactions. We will present how the resulting quantum many-body interactions enhances spatial bunching of the atoms which can be literally seen as dramatic, macroscopically-visible changes in the interference pattern. Consequently, future experiments should easily access many-body correlations via such an imaging.