EMBARGOED UNTIL 00:01 (BST) WEDNESDAY 15 JULY 2015 Review - Quantum optics with one or two photons Proceedings of the Royal Society A: Mathematical and Physical Sciences The concept of a photon is central to our understanding of the quantum world. We summarise our modern understanding of a photon and describe a number of new technologies based on single photon states of light with an emphasis on optomechanical systems. Contact: Dr Gerard Milburn, The University of Queensland, Milburn@physics.uq.edu.au, 61733656931 URL after publication: http://rspa.royalsocietypublishing.org/lookup/doi/10.1098/rspa.2015.0208