Witnessing quantumness of a system by observing only its classical features

Understand

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.