Cavity QED: Nonlinearity and the World of Quanta (joint colloquium with the Department of Physics and Astronomy) - Howard Carmichael

The invention of the laser in the early 1960s introduced physics to the world of optical nonlinearity. Indeed, laser action itself exemplifies that nonlinearity, with its sustained oscillation above threshold emerging in the manner of a second-order phase transition. Though numbers vary, something like 104 to 108 photons are needed to “turn on” the nonlinearity of a typical laser. Imagine then that one photon would do it! This exercise defines cavity quantum electrodynamics (cavity QED), where one visible photon, confined in a small enough cavity, might produce an electric field of 10, 100, or even 1000 V/cm. My talk reviews 20 years of progress in cavity QED, focusing on a comparison of linear response, which is largely classical, and those nonlinear (multi-photon) effects that tell an unequivocal story of the quantized nature of light. The history tracks past recent dramatic experiments that relate the story with microwave photons and a superconducting qubit, to the prediction of a novel quantum phase transition for photons, where quantum trajectories tell the story…one of a distinctly quantum “nonlinear dynamics”.