# Quantum correlations in composite systems

## TL;DR

This paper explores various quantum correlations in composite systems, analyzing their definitions, representations, and relationships, including entanglement and other nonclassical correlations, with implications for multimode and temporal quantum systems.

## Contribution

It introduces a unified framework for understanding quantum correlations, connecting concepts from quantum optics and quantum information, and highlights the coexistence of entanglement with other quantum correlations.

## Key findings

- Entanglement is a subset of quantum correlations in the framework.
- Non-entanglement quantum correlations can exist independently.
- Representation via quasiprobability matrices aids in understanding correlations.

## Abstract

We study emerging notions of quantum correlations in compound systems. Based on different definitions of quantumness in individual subsystems, we investigate how they extend to the joint description of a composite system. Especially, we study the bipartite case and the connection of two typically applied and distinctively different concepts of nonclassicality in quantum optics and quantum information. Our investigation includes the representation of correlated states in terms of quasiprobability matrices, a comparative study of joint and conditional quantum correlations, and an entanglement characterization. It is, for example, shown that our composition approach always includes entanglement as one form of quantum correlations. Yet, other forms of quantum correlations can also occur without entanglement. Finally, we give an outlook towards multimode systems and temporal correlations.