# Witnessing quantumness of a system by observing only its classical features

## TL;DR

This paper proposes a thought experiment to witness non-classicality in a system by probing it with a qubit through classical measurements, potentially enabling laboratory detection of quantum gravitational effects.

## Contribution

It introduces a method to detect non-classicality without requiring quantum control of the system or specific dynamics, using only classical observables and a qubit probe.

## Key findings

- Non-classicality can be witnessed indirectly via classical measurements.
- The scheme does not depend on the system's specific dynamics.
- Potential to observe quantum gravitational effects in laboratory settings.

## Abstract

Witnessing non-classicality in the gravitational field has been claimed to be practically impossible. This constitutes a deep problem, which has even lead some researchers to question whether gravity should be quantised, due to the weakness of quantum effects. To counteract these claims, we propose a thought experiment that witnesses non-classicality of a physical system by probing it with a qubit. Remarkably, this experiment does not require any quantum control of the system, involving only measuring a single classical observable on that system. In addition, our scheme does not even assume any specific dynamics. That non-classicality of a system can be established indirectly, by coupling it to a qubit, opens up the possibility that quantum gravitational effects could in fact be witnessed in the lab.