  It is possible to make a detector which clicks after absorbing a single
graviton. Similarly, it is possible to make a gravitational wave detector which
can see the quantum noise induced by certain highly squeezed states of the
graviton. However, contrary to some recent arguments in the literature,
observation of either or both of these signals would not constitute proof or
even evidence that the gravitational field is quantized. This is a simple
technical statement: a classical gravitational wave can produce the same output
data in a detector. Here we explain this result, presented earlier in
arXiv:2308.12988, which is a straightforward extension of an ancient argument
in quantum optics.
