Nonlinear optics and quantum entanglement of ultra-slow photons

Summary form only given. Exciting observations of ultra-slow group velocities and very large nonlinearities in coherently driven atomic media open up interesting prospectives for nonlinear optical processes at low light levels. Recent experiments, for instance, demonstrated that very efficient nonlinear phenomena can be induced by laser fields of tiny energies. These developments stimulate the studies of ultimate efficiency limits for nonlinear optics based on electromagnetically induced transparency (EIT) as well as its possible applications for generation and manipulation of quantum entangled states of the light fields and matter. We describe a particular method that allows for two slow light fields of energies corresponding to that of a "single photon" to interact coherently.