Merging of Rotating Bose–Einstein Condensates

Merging of isolated Bose–Einstein condensates (BECs) is an important topic due to its relevance to matter-wave interferometry and the Kibble–Zurek mechanism. Many past research focused on merging of BECs with uniform initial phases. In our recent brief report (Kanai et al. in Phys Rev A 97:013612, 2018), we showed that upon merging of rotating BECs with non-uniform initial phases, spiral-shaped dark solitons can emerge. These solitons facilitate angular momentum transfer and allow the merged condensate to rotate even in the absence of quantized vortices. More strikingly, the sharp endpoints of these spiral solitons can induce rotational motion in the BECs like vortices but with effectively a fraction of a quantized circulation. This paper reports our systematic study on the merging dynamics of rotating BECs. We discuss how the relative winding number of the rotating BECs and the potential barrier that initially separates the BECs may affect the profile and dynamics of the spiral solitons. The number of spiral solitons created in the BECs is observed to always match exactly the relative winding number of the two BECs. The underlying mechanism for which the solitons can break up to form sharp endpoints with peculiar physical properties and why the number of solitons matches the relative winding number is identified and explained. These results improve our understanding of soliton dynamics, which may allow better manipulation of these non-topological phase defects when they are involved in various quantum transport processes.