Propagation and Localization of Collective Excitations on a 24-Qubit Superconducting Processor

Superconducting circuits have emerged as a powerful platform of quantum simulation, especially for emulating the dynamics of quantum many-body systems. Here we construct a Bose-Hubbard ladder with a ladder array of 20 qubits. We then use pairs of controllable qubits to study the dynamics of the ladder model. We investigate theoretically and demonstrate experimentally the dynamics of single- and double-excitation states with distinct behaviors. We observe linear propagation of photons in the single-excitation case. The double-excitation state, initially placed at the edge, localizes; while placed in the bulk, it splits into two single-excitation modes. Our work paves the way to simulation of exotic logic particles by subtly encoding physical qubits and exploration of rich physics by superconducting circuits.