Eavesdropping on the Decohering Environment: Quantum Darwinism, Amplification, and the Origin of Objective Classical Reality

TL;DR

This paper investigates how information about a quantum system becomes accessible through its environment, analyzing the limits of information extraction and the emergence of classical objectivity using a model with imperfect quantum gates.

Contribution

It introduces a model with imperfect c-not gates to compute accessible information and related bounds, revealing scale-invariant behavior relevant to classical reality emergence.

Findings

Quantum mutual information and bounds exhibit similar behavior.

Information quantities scale independently of gate quality and environment size.

Emergence of classical objectivity is nearly independent of the system's initial state.

Abstract

"How much information about a system $\mathcal{S}$ can one extract from a fragment $\mathcal{F}$ of the environment $\mathcal{E}$ that decohered it?" is the central question of Quantum Darwinism. To date, most answers relied on the quantum mutual information of $\mathcal{SF}$, or on the Holevo bound on the channel capacity of $\mathcal{F}$ to communicate the classical information encoded in $\mathcal{S}$. These are reasonable upper bounds on what is really needed but much harder to calculate -- the accessible information in the fragment $\mathcal{F}$ about $\mathcal{S}$. We consider a model based on imperfect c-not gates where all the above can be computed, and discuss its implications for the emergence of objective classical reality. We find that all relevant quantities, such as the quantum mutual information as well as various bounds on the accessible information exhibit similar behavior. In the regime relevant for the emergence of objective classical reality this includes scaling independent of the quality of the imperfect c-not gates or the size of $\mathcal{E}$, and even nearly independent of the initial state of $\mathcal{S}$.