  We show that when the gravitational field is treated quantum-mechanically, it
induces fluctuations -- noise -- in the lengths of the arms of gravitational
wave detectors. The characteristics of the noise depend on the quantum state of
the gravitational field, and can be calculated exactly in several interesting
cases. For coherent states the noise is very small, but it can be greatly
enhanced in thermal and (especially) squeezed states. Detection of this
fundamental noise would constitute direct evidence for the quantization of
gravity and the existence of gravitons.
