“…It is well known that, classically, the inverted oscillator has an unstable fixed point in phase space at (x = 0, p = 0) (and, hence, not chaotic in the strict sense). Nonetheless, in the context of studying quantum chaos in various quantum field theories [48][49][50][51][52][53][54][55], it has served as a powerful toy model mostly because it is an exactly solvable system. We begin by revisiting the complexity of the doubly-evolved circuit state, namely a state obtained by first evolving the system forward in time with a Hamiltonian H, and then evolving it backwards in time with a Hamiltonian H + δH.…”