# Revealing correlations between a system and an inaccessible environment

## TL;DR

This paper reviews a local detection method for identifying correlations between a controllable quantum system and an inaccessible environment, supported by recent experimental demonstrations with photons and trapped ions.

## Contribution

It summarizes the theoretical framework and recent experimental progress in detecting system-environment correlations without direct access to the environment.

## Key findings

- Successful experimental detection with single photons
- Implementation with trapped ions
- Validation of the local detection method

## Abstract

How can we detect that our local, controllable quantum system is correlated with some other inaccessible environmental system? The local detection method developed in recent years allows to realize a dynamical witness for correlations without requiring knowledge of or access to the environment that is correlated with the local accessible quantum system. Here, we provide a brief summary of the theoretical method and recent experimental studies with single photons and trapped ions coupled to increasingly complex environments.