  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.
