Direct observation of geometric-phase interference in dynamics around a conical intersection*

Conical intersections in molecular systems are geometry points where two potential energy surfaces have the same energy and govern processes such as light harvesting, vision, photocatalysis, and chemical reactivity.They act as funnels between electronic states of molecules, allowing rapid and efficient relaxation during molecular dynamics. In addition, when a nuclear wavepacket encircles a conical intersection, the wavefunction experiences a geometric phase, which affects the outcome of the reaction through quantum-mechanical interference. Past experiments have measured indirect signatures of geometric phases in scattering patterns and spectroscopic observables, but there has been no direct observation of the underlying wavepacket interference. Here, we experimentally observe geometric-phase interference in the dynamics of a nuclear wavepacket travelling around an engineered conical intersection in a programmable trapped-ion quantum simulator. To achieve this, we develop a new technique to reconstruct the two-dimensional wavepacket densities of a trapped ion. Experiments agree with the theoretical model, demonstrating the ability of analog quantum simulators---such as those realised using trapped ions---to accurately describe nuclear quantum effects. These results demonstrate a path to deploying analog quantum simulators for solving some of the most difficult problems in molecular dynamics.