Entanglement: Quantum or Classical?

TL;DR

This paper explores the fundamental differences between quantum and classical entanglement, reviewing historical context, hidden variables models, and experimental violations of classical bounds to clarify the uniquely quantum nature of entanglement.

Contribution

It provides a comprehensive analysis of classical explanations for entanglement, discusses inequalities violated by quantum states, and reviews recent advances and future directions in quantum entanglement research.

Findings

Quantum states violate classical inequalities, falsifying hidden variables theories.

Entanglement manifests in phenomena like N00N states and non-separable single particles.

Historical and theoretical analysis clarifies the quantum nature of entanglement.

Abstract

From its seemingly non-intuitive and puzzling nature, most evident in numerous EPR-like gedankenexperiments to its almost ubiquitous presence in quantum technologies, entanglement is at the heart of modern quantum physics. First introduced by Erwin Schr\"{o}dinger nearly a century ago, entanglement has remained one of the most fascinating ideas that came out of quantum mechanics. Here, we attempt to explain what makes entanglement fundamentally different from any classical phenomenon. To this end, we start with a historical overview of entanglement and discuss several hidden variables models that were conceived to provide a classical explanation and demystify quantum entanglement. We discuss some inequalities and bounds that are violated by quantum states thereby falsifying the existence of some of the classical hidden variables theories. We also discuss some exciting manifestations of entanglement, such as N00N states and the non-separable single particle states. We conclude by discussing some contemporary results regarding quantum correlations and present a future outlook for the research of quantum entanglement.