Interference of longitudinal and transversal fragmentations in the Josephson tunneling dynamics of Bose-Einstein condensates

The dynamics of bosons in Josephson junctions have drawn much attention where the bosons are initially condensed. When interacting bosons tunnel back and forth along the junction, depletion and, eventually, fragmentation develop. Here, we pose the question how do fragmented bosons tunnel in a bosonic Josephson junction? To this end, we exploit the transverse degree-of-freedom of the junction to encode initial fragmentation to the bosonic cloud. We find rich many-body dynamics, which includes the speed up of the collapse of density oscillations and slow down of the revival process, by analyzing the survival probability along the junction, fluctuations of particle positions across the junction, and the occupancy of the lowest single-particle states. It is found that a fully fragmented state significantly accelerates the revival process compared to the conventional Bose-Einstein condensate. To explain the underlying many-body mechanism, we show that the initial fragmentation in the transverse direction interferes with the development of fragmentation in time along the junction. The magnitude of this interference is quantified by the transition from a decrease of occupation to build up of occupation in the first excited single-particle state. The interference mechanism is a purely many-body effect that does not occur in the mean-field dynamics. All in all, we show that the interference of longitudinal and transversal fragmentations leads to new rules for macroscopic tunneling phenomena of interacting bosons in traps.