Complexity from the reduced density matrix: a new diagnostic for chaos

We investigate circuit complexity to characterize chaos in multiparticle quantum systems; concomitantly, we take strides in using complexity to characterize open quantum systems. By exploring different types of quantum circuits, we propose a new diagnostic of chaos from complexity based on the reduced density matrix. Through explicit calculations on a model of two coupled harmonic oscillators where one or both of the oscillators are inverted, we demonstrate that the evolution of complexity is an effective diagnostic of chaos.