Non-reciprocal dynamics and the non-Hermitian skin effect of repulsively bound pairs

We study the dynamics of a Bose-Hubbard model coupled to an engineered environment which in the non-interacting limit is described by the celebrated Hatano-Nelson model. At strong interactions, two bosons occupying the same site form a so-called repulsively bound pair, or doublon. Using tensor-network simulations, we clearly identify a distinct doublon lightcone and show that the doublon inherits non-reciprocity from that of single particles. Applying the idea of reservoir engineering at the level of doublons, we introduce a new set of dissipators and we analytically show that then the doublon dynamics are governed by the Hatano-Nelson model. This brings about an interaction-induced non-Hermitian skin effect and non-reciprocal doublon motion. Combining features of the two models we study, we show that single particles and doublons can be made to spread with opposite directionality, opening intriguing possibilities for the study of dynamics in interacting non-reciprocal models.