Fast-Scrambling and Operator Confinement Using an Auxiliary Qubit

TL;DR

This paper presents a minimal model with an auxiliary qubit that demonstrates a transition from fast to slow quantum information scrambling, revealing mechanisms for operator confinement and entanglement growth suppression.

Contribution

It introduces a new model showing how an auxiliary qubit mediates a transition from super-ballistic to sub-ballistic scrambling and operator confinement, with exact dynamics and analytic expansions.

Findings

Transition from super-ballistic to sub-ballistic scrambling

Logarithmic entanglement growth at slow scrambling regime

Operators orthogonal to the interaction become echoed out

Abstract

We introduce a minimal model for realizing a fast-to-slow scrambling transition mediated by an auxiliary central qubit (c-qubit). The c-qubit is coupled to a spin-$1/2$ Ising model with local Ising interactions and tunable c-qubit-spin coupling. Each spin becomes next-nearest neighbor to all others through the c-qubit, which mediates effective all-to-all interactions. As the interaction with the c-spin increases, we find a surprising transition from super-ballistic scrambling and information growth to continuously restricted sub-ballistic entanglement and operator growth. This slow growth occurs on intermediate timescales that extend exponentially with increasing coupling and system size, indicative of logarithmic entanglement growth. We find that in the slow-scrambling regime, the c-qubit Ising interaction allows commuting operators to grow support on all sites rapidly, while operators orthogonal to the interaction become echoed out. This projects local operators to lie in a restricted subspace and prevents extensive operator entanglement growth. We provide exact dynamics of small systems working with non-equilibrium, effective infinite temperature states, and additionally contribute analytic early-time expansions that support the observed rapid scrambling to quantum Zeno-like crossover. Tracing out the central qubit provides a unique translation from the full, closed unitary dynamics to a simple open system construction consisting of a typical spin-chain with hidden qubit degree of freedom.