Possible Graviton Transitions and Gaser Action in High-Tc   Superconductors
  It is well known that excited quantum systems can emit gravitons as well as photons. Differently from photon emission, spontaneous graviton emission is characterized by such a low probability that stimulated emission appears the only possible mechanism for producing gravitational radiation with quantum systems. The principles of the gravitational counterpart of the laser, which has been named gaser, have been theorized decades ago. Now a class of high temperature superconductors might be identified as a viable active material for practical gaser action. 
Giorgio Fontana