Physics Colloquium (16/06/2022). Abstract: The enduring challenge of reconciling quantum mechanics and general relativity is not helped by a lack of experimental guidance. Theoretical reconciliations, such as loop quantum gravity and string theory, seem to require unachievable experimental domains, and, while astrophysical data may one day provide guidance, the possibility of lab based experiments seems remote. Recently there have been suggestions for lab based tests of one aspect of the problem: gravitational decoherence. In this talk, I will review the concept of gravitational decoherence form Karolyhazy to Penrose and Diosi. I will discuss opto-mechanical proposals for constraining the decoherence rate by experiment. I will conclude with a discussion of a new approach based on single photonics that avoid many of the challenges that arise from tests involving particles with large rest mass.