  We revisit a question asked by Dyson: "Is a graviton detectable?" We
demonstrate that in both Dyson's original sense and in a more modern
measurement-theoretic sense, it is possible to construct a detector sensitive
to single gravitons, and in fact a variety of existing and near-term
gravitational wave detectors can achieve this. However, while such a signal
would be consistent with the quantization of the gravitational field, we draw
on results from quantum optics to show how the same signal could just as well
be explained via classical gravitational waves. We outline the kind of
measurements that would be needed to demonstrate quantization of gravitational
radiation and explain why these are substantially more difficult than simply
counting graviton clicks or observing gravitational noise in an interferometer,
and likely impossible to perform in practice.
