Classical approach to equilibrium of out-of-time ordered correlators in mixed systems

The out-of-time ordered correlator (OTOC) is a measure of scrambling of quantum information. Scrambling is intuitively considered to be a significant feature of chaotic systems and thus the OTOC is widely used as a measure of chaos. For short times exponential growth is related to the classical Lyapunov exponent, sometimes known as butterfly effect. At long times the OTOC attains an average equilibrium value with possible oscillations. For fully chaotic systems the approach to the asymptotic regime is exponential with a rate given by the classical Ruelle-Pollicott resonances. In this work, we extend this notion by showing that classical generalized resonances govern the relaxation to equilibrium of the OTOC in the ubiquitous case of a system with mixed dynamics, in particular, the standard map.