A Bound on Chaos from Stability

Abstract

We explore the quantum chaos of the coadjoint orbit action. We study quantum fluctuation around a saddle point to evaluate the soft mode contribution to the out-of-time-ordered correlator. We show that the stability condition of the semi-classical analysis of the coadjoint orbit found by Witten (1988) leads to the upper bound on the Lyapunov exponent which is identical to the bound on chaos proven in arXiv:1503.01409. The bound is saturated by the coadjoint orbit $\text{Diff}(S^1)/SL(2)$ while the other stable orbit $\text{Diff}(S^1)/U(1)$ where the $SL(2,\mathbb{R})$ is broken to $U(1)$ has non-maximal Lyapunov exponent.