Dark–bright soliton dynamics beyond the mean-field approximation

Abstract

The dynamics of dark-bright (DB) solitons beyond the mean-field approximation is investigated. We first examine the case of a single DB soliton and its oscillations within a parabolic trap. Subsequently, we move to the setting of collisions, comparing the mean-field approximation to that involving multiple orbitals in both the dark and the bright component. Fragmentation is present and significantly affects the dynamics, especially in the case of slower solitons and in that of lower atom numbers. It is shown that the presence of fragmentation allows for bipartite entanglement between the distinguishable species. Most importantly the interplay between fragmentation and entanglement leads to the splitting of each of the parent mean-field DB solitons, placed off-center within the parabolic trap, into a fast and a slow daughter solitary wave. The latter process is in direct contrast to the predictions of the mean-field approximation. A variety of excitations including DB solitons in multiple (concurrently populated) orbitals is observed. Dark-antidark states and domain-wall-bright soliton complexes can also be observed to arise spontaneously in the beyond mean-field dynamics. that slower solitons and smaller atom numbers result in significant deviations from the mean-field, Gross-Pitaevskii (GP) limit.However, to the best of our knowledge, such studies have not been performed in a systematic fashion in multi-component settings and for associated solitary wave structures. In that light, herein we explore the case of DB solitons and their dynamics, as well as collisions both at and beyond the mean-field limit. To incorporate the quantum fluctuations stemming from the correlations in the DB soliton dynamics, we employ the multi-layer multi-configuration time-dependent Hartree method for bosons (ML-MCTDHB) [47,48] designed for simulating the quantum dynamics of bosonic mixtures. We consider both the oscillation of a single DB solitary wave in a trap, as well as the interaction of two symmetric solitary waves inside a parabolic trap. We compare and contrast the findings of the mean-field case (where a single orbital is effectively used in the dark-and brightcomponents) with cases where multiple orbitals are used. In all cases it is found that the initial mean-field DB solitons split into daughter DB solitary waves, in contrast to the well-known mean-field predictions. The robustness of the presented results is ensured by exploring settings that involve a higher number of orbitals thereby supporting the validity of our approximation and of the observed beyond mean-field excitations (see also appendix B). Within the employed multi-orbital approximation bipartite entanglement (see [49,50] and references therein) between the distinguishable species resulting from the spontaneous fragmentation of the DBs is generally present. More importantly it is the interplay between fragmentation and the resulting entanglement which gives rise to the observed dynamical structures.The dynamics of a single DB soliton being initialized off-ce...

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