Bright-soliton quantum superpositions: Signatures of high- and low-fidelity states

Scattering quantum bright solitons off barriers has been predicted to lead to nonlocal quantum superpositions, in particular the NOON state. The focus of this paper lies on signatures of both high-and low-fidelity quantum superposition states. We numerically demonstrate that a one-dimensional geometry with the barrier potential situated in the middle of an additional-experimentally typical-harmonic confinement gives rise to particularly well-observable signatures. In the elastic-scattering regime we investigate signatures of NOON states on the N-particle level within an effective potential approach. We show that removing the barrier potential and subsequently recombining both parts of the quantum superposition leads to a high-contrast interference pattern in the center-of-mass coordinate for narrow and broad potential barriers. We demonstrate that the presented signatures can be used to clearly distinguish quantum superposition states from statistical mixtures and are sufficiently robust against experimentally relevant excitations of the center-of-mass wave function to higher-lying oscillator states. For two-particle solitons we extend these considerations to low-fidelity superposition states: even for strong deviations from the two-particle NOON state we find interference patterns with high contrast.