Electrical Switching of the Berry Phase in Bernal Bilayer Graphene Quantum Dots.

The Berry phase-a fundamental quantum geometric phase given by the integral of the Berry curvature over a closed trajectory in momentum space-underlies a wide range of quantum topological phenomena. Recent advances have demonstrated the potential of tuning the Berry phase to access exotic quantum states, primarily by modifying the integration path. However, its definition also suggests an alternative, yet largely unexplored route: the direct control of the Berry curvature itself. Here, we realize continuous modulation of the Berry curvature in the Bernal-stacked bilayer graphene quantum dot by applying a vertical displacement field. This tuning alters the Berry phase accumulated along confined electron orbits, leading to a distinct energy splitting between the M=0 and M=1 angular momentum states. Moreover, we demonstrate that the split states exhibit characteristics of atomic collapse states, finally resolving a long-standing academic debate about their existence in Bernal-stacked bilayer graphene. These observations establish the Berry curvature as a robust, magnetic field-free control parameter for manipulating confined states, unveiling a previously unobserved link between band topology and the evolution of quantum-confined states.