Scrambling and quantum feedback in a nanomechanical system

TL;DR

This paper investigates how quantum feedback influences entanglement spreading in coupled nanomechanical systems with NV center spins, using the out-of-time ordered correlator (OTOC) to quantify quantum correlations and feedback effects.

Contribution

It introduces a model of coupled nanomechanical oscillators with NV spins to analyze quantum feedback and entanglement propagation using OTOC, highlighting differences between quantum and classical oscillators.

Findings

NV spins can exert quantum feedback only in quantum regimes

OTOC between NV spins measures feedback strength

Classical oscillators do not support quantum feedback

Abstract

The question of how swiftly entanglement spreads over a system has attracted vital interest. In this regard, the out-of-time ordered correlator (OTOC) is a quantitative measure of the entanglement spreading process. Particular interest concerns the propagation of quantum correlations in the lattice systems, {\it e.g.}, spin chains. In a seminal paper D. A. Roberts, D. Stanford and L. Susskind, J. High Energy Phys. 03, 051, (2015) the concept of the OTOC's radius was introduced. The radius of the OTOC defines the front line reached by the spread of entanglement. Beyond this radius operators commute. In the present work, we propose a model of two nanomechanical systems coupled with two Nitrogen-vacancy (NV) center spins. Oscillators are coupled to each other directly while NV spins are not. Therefore, the correlation between the NV spins may arise only through the quantum feedback exerted from the first NV spin to the first oscillator and transferred from the first oscillator to the second oscillator via the direct coupling. Thus nonzero OTOC between NV spins quantifies the strength of the quantum feedback. We show that NV spins cannot exert quantum feedback on classical nonlinear oscillators. We also discuss the inherently quantum case with a linear quantum harmonic oscillator indirectly coupling the two spins and verify that in the classical limit of the oscillator, the OTOC vanishes.