  All existing quantum gravity proposals share the same deep problem. Their
predictions are extremely hard to test in practice. Quantum effects in the
gravitational field are exceptionally small, unlike those in the
electromagnetic field. The fundamental reason is that the gravitational
coupling constant is about 43 orders of magnitude smaller than the fine
structure constant, which governs light-matter interactions. For example, the
detection of gravitons -- the hypothetical quanta of energy of the
gravitational field predicted by certain quantum-gravity proposals -- is deemed
to be practically impossible. In this letter we adopt a radically different,
quantum-information-theoretic approach which circumvents the problem that
quantum gravity is hard to test. We propose an experiment to witness
quantum-like features in the gravitational field, by probing it with two masses
each in a superposition of two locations. First, we prove the fact that any
system (e.g. a field) capable of mediating entanglement between two quantum
systems must itself be quantum. This argument is general and does not rely on
any specific dynamics. Then, we propose an experiment to detect the
entanglement generated between two masses via gravitational interaction. By our
argument, the degree of entanglement between the masses is an indirect witness
of the quantisation of the field mediating the interaction. Remarkably, this
experiment does not require any quantum control over gravity itself. It is also
closer to realisation than other proposals, such as detecting gravitons or
detecting quantum gravitational vacuum fluctuations.
