# Quantum correlations in separable multi-mode states and in classically entangled light

## TL;DR

This review explores quantum-like properties of classical optical fields, focusing on quantum correlations, discord, and entanglement in multi-mode and vector light fields, highlighting their potential in quantum technologies.

## Contribution

It clarifies the quantum features of classically perceived systems, emphasizing quantum discord, classical entanglement, and intra-system correlations for quantum applications.

## Key findings

- Quantum discord enables entanglement activation.
- Classical entanglement can be used for high-precision measurements.
- Intra-system correlations facilitate quantum information tasks.

## Abstract

In this review we discuss intriguing properties of apparently classical optical fields, that go beyond purely classical context and allow us to speak about quantum characteristics of such fields and about their applications in quantum technologies. We briefly define the genuinely quantum concepts of entanglement and steering. We then move to the boarder line between classical and quantum world introducing quantum discord, a more general concept of quantum coherence, and finally a controversial notion of classical entanglement. To unveil the quantum aspects of often classically perceived systems, we focus more in detail on quantum discordant correlations between the light modes and on nonseparability properties of optical vector fields leading to entanglement between different degrees of freedom of a single beam. To illustrate the aptitude of different types of correlated systems to act as quantum or quantum-like resource, entanglement activation from discord, high-precision measurements with classical entanglement and quantum information tasks using intra-system correlations are discussed. The common themes behind the versatile quantum properties of seemingly classical light are coherence, polarization and inter and intra--mode quantum correlations.