  The task of testing whether quantum theory applies to all physical systems
and all scales requires considering situations where a quantum probe interacts
with another system that need not obey quantum theory in full. Important
examples include the cases where a quantum mass probes the gravitational field,
for which a unique quantum theory of gravity does not yet exist, or a quantum
field, such as light, interacts with a macroscopic system, such as a biological
molecule, which may or may not obey unitary quantum theory. In this context a
class of experiments has recently been proposed, where the non-classicality of
a physical system that need not obey quantum theory (the gravitational field)
can be tested indirectly by detecting whether or not the system is capable of
entangling two quantum probes. Here we illustrate some of the subtleties of the
argument, to do with the role of locality of interactions and of
non-classicality, and perform proof-of-principle experiments illustrating the
logic of the proposals, using a Nuclear Magnetic Resonance quantum
computational platform with four qubits.
