Quantum superposition of topological defects in ion trap system

Topological defects are discontinuities of a system in phase transitions and have been explored in condensed matter physics and cosmology. Numerous experimental evidences have confirmed their existence and revealed their classical properties such as density. While quantum superposition of topological defects has been predicted, observing this phenomenon experimentally still remains a challenge despite their fundamental importance. Here, we report the observation of quantum superposition of topological defects in a trapped-ion system. By engineering the long-range spin-spin interactions, we observe a single topological defect, namely a spin kink, splitting into a superposition of kinks at different positions, creating a ``Schrodinger kink'' that manifests non-locality and quantum interference. Furthermore, by preparing superposition states of neighboring kinks with different phases, we observe the propagation of the wave packet in different directions, thus unambiguously verifying the quantum coherence in the superposition states. Our work provides useful tools for non-equilibrium dynamics in quantum Kibble-Zurek physics.