Global Out of Time Order Correlators as a Signature of Scrambling Dynamics of Local Observables

TL;DR

This paper demonstrates that in large, homogeneous quantum systems, global out-of-time-order correlators (OTOCs) can be accurately approximated by local OTOCs, simplifying the measurement of quantum information scrambling.

Contribution

It establishes a direct connection between global and local OTOCs in NMR experiments and shows their equivalence in large systems, advancing understanding of quantum scrambling detection.

Findings

Global and local OTOCs become equivalent as system size increases.

Local OTOCs determine global OTOCs after initial transient.

Fluctuations between local and global OTOCs diminish with system size.

Abstract

Out-of-Time-Order Correlators (OTOCs) serve as a proxy for quantum information scrambling, which refers to the process where information stored locally disperses across the many-body degrees of freedom in a quantum system, rendering it inaccessible to local probes. Most experimental implementations of OTOCs to probe information scrambling rely on indirect measurements based on global observables, using techniques such as Loschmidt echoes and Multiple Quantum Coherences, via time reversal evolutions. In this article, we establish a direct connection between OTOCs with global and local observables in the context of NMR experiments, where the observable is the total magnetization of the system. We conduct a numerical analysis to quantify the differences in the evolution of both magnitudes, evaluating the excitation dynamics in spin ring systems with 8 to 16 spins, using a many-body Hamiltonian and long-range interactions. Our analysis decomposes the global echo into a sum of local echoes and cross-contributions, leading to local and global OTOCs. The results indicate that, after an initial transient period, local OTOCs determine the global ones. We observe that the difference between the average of local OTOCs and the global one, as well as their fluctuations, becomes negligible as the system size increases. Thus, for large homogeneous systems, global and local OTOCs become equivalent. This behavior aligns with that observed in highly interacting or chaotic systems in several experiments.