Extracting classical Lyapunov exponent from one-dimensional quantum mechanics

TL;DR

This paper demonstrates that out-of-time-order correlators (OTOCs) in one-dimensional quantum mechanics with polynomial potentials can reproduce classical Lyapunov exponents, revealing quantum signatures of chaos.

Contribution

It extends the understanding of quantum chaos by showing how OTOCs in polynomial potentials reflect classical Lyapunov exponents, especially near potential peaks.

Findings

OTOCs exhibit exponential growth matching classical Lyapunov exponents.

Localized wave packets near the potential peak reveal classical butterfly effect.

Large-N-like limits also reproduce classical chaos signatures.

Abstract

The commutator $[x(t),p]$ in an inverted harmonic oscillator (IHO) in one-dimensional quantum mechanics exhibits remarkable properties. It reduces to a c-number and does not show any quantum fluctuations for arbitrary states. Related to this nature, the quantum Lyapunov exponent computed through the out-of-time-order correlator (OTOC) $\langle [x(t),p]^2 \rangle $ precisely agrees with the classical one. Hence, the OTOC may be regarded as an ideal indicator of the butterfly effect in the IHO. Since IHOs are ubiquitous in physics, these properties of the commutator $[x(t),p]$ and the OTOCs might be seen in various situations, too. In order to clarify this point, as a first step, we investigate OTOCs in one-dimensional quantum mechanics with polynomial potentials, which exhibit butterfly effects around the peak of the potential in classical mechanics. We find two situations in which the OTOCs show exponential growth reproducing the classical Lyapunov exponent of the peak. The first one, which is obvious, is using a suitably localized wave packet near the peak, and the second one is taking a limit akin to the large-$N$ limit in the noncritical string theories.