  The quantization of gravity is widely believed to result in gravitons --
particles of discrete energy that form gravitational waves. But their detection
has so far been considered impossible. Here we show that signatures of single
gravitons can be observed in laboratory experiments. We show that stimulated
and spontaneous single-graviton processes can become relevant for massive
quantum acoustic resonators and that stimulated absorption can be resolved
through continuous sensing of quantum jumps. We analyze the feasibility of
observing the exchange of single energy quanta between matter and gravitational
waves. Our results show that single graviton signatures are within reach of
experiments. In analogy to the discovery of the photo-electric effect for
photons, such signatures can provide the first experimental evidence of the
quantization of gravity.
