Timescales in the quench dynamics of many-body quantum systems: Participation ratio versus out-of-time ordered correlator.

We study quench dynamics in the many-body Hilbert space using two isolated systems with a finite number of interacting particles: a paradigmatic model of randomly interacting bosons and a dynamical (clean) model of interacting spins-1/2. For both systems in the region of strong quantum chaos, the number of components of the evolving wave function, defined through the number of principal components N_{pc} (or participation ratio), was recently found to increase exponentially fast in time [Phys. Rev. E 99, 010101(R) (2019)2470-004510.1103/PhysRevE.99.010101]. Here, we ask whether the out-of-time ordered correlator (OTOC), which is nowadays widely used to quantify instability in quantum systems, can manifest analogous time dependence. We show that N_{pc} can be formally expressed as the inverse of the sum of all OTOCs for projection operators. While none of the individual projection OTOCs show an exponential behavior, their sum decreases exponentially fast in time. The comparison between the behavior of the OTOC with that of the N_{pc} helps us better understand wave packet dynamics in the many-body Hilbert space, in close connection with the problems of thermalization and information scrambling.