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Macroscopic quantum self-trapping is a nonlinear regime of a bosonic Josephson junction in which an initial population imbalance does not undergo the full sign-changing tunneling oscillations expected from a simple linear two-state model. Interactions alter the relation between relative phase and population flow.

An experiment with two weakly linked condensates in a double-well potential measured both the density distribution and the relative phase inferred from interference fringes. Their combination distinguished nonlinear self-trapping from merely observing unequal populations at one instant.^[ref]

An experimental atomic physicist who coauthored the realization of a single bosonic Josephson junction from two weakly coupled Bose-Einstein condensates in a double-well potential. The experiment measured the density in each well and inferred the relative phase from interference fringes. It distinguished oscillatory transfer from interaction-induced macroscopic quantum self-trapping, where an initial population imbalance persists despite the coupling.^[ref]