Detecting single gravitons with quantum-controlled mechanical oscillators

 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 optomechanical read-out of single phonons of a multi-mode bar resonator. We analyse the feasibility of observing a signal from the inspiral, merger and post-merger phase of a compact binary inspiral. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photoelectric effect for photons, such signatures can provide the first experimental evidence of the quantization of gravity.