Large optical nonlinearities with few photons

Weak-light all-optical switches would enhance photonics capabilities and enable important optical quantum information processing tasks, but optical switching based on third-order optical nonlinearity is proving to be elusive and controversial. I discuss strategies to generate large cross-phase modulation by combining double electromagnetically-induced transparency, competing atomic transitions, electron population engineering for pulse speed control, and using boundary conditions to confine the transverse field beyond the diffraction limit.