# Crossing Over from Attractive to Repulsive Interactions in a Tunneling Bosonic Josephson Junction

## TL;DR

This paper investigates the transition from attractive to repulsive interactions in a bosonic Josephson junction using a tunable $^{39}$K Bose-Einstein condensate, revealing various dynamical regimes and a quantum phase transition.

## Contribution

It demonstrates control over interaction regimes in a bosonic Josephson junction and observes a quantum phase transition with coherent dynamics, expanding understanding of many-body quantum systems.

## Key findings

- Observation of critical slowing down near the quantum phase transition
- Detection of macroscopic quantum self-trapping with coherent oscillations
- Broad agreement between experimental results and theoretical models

## Abstract

We explore the interplay between tunneling and interatomic interactions in the dynamics of a bosonic Josephson junction. We tune the scattering length of an atomic $^{39}$K Bose-Einstein condensate confined in a double-well trap to investigate regimes inaccessible to other superconducting or superfluid systems. In the limit of small-amplitude oscillations, we study the transition from Rabi to plasma oscillations by crossing over from attractive to repulsive interatomic interactions. We observe a critical slowing down in the oscillation frequency by increasing the strength of an attractive interaction up to the point of a quantum phase transition. With sufficiently large initial oscillation amplitude and repulsive interactions the system enters the macroscopic quantum self-trapping regime, where we observe coherent undamped oscillations with a self-sustained average imbalance of the relative well population. The exquisite agreement between theory and experiments enables the observation of a broad range of many body coherent dynamical regimes driven by tunable tunneling energy, interactions and external forces, with applications spanning from atomtronics to quantum metrology.