Nonlinear optics and quantum entanglement of ultraslow single photons

Two light pulses propagating with slow group velocities in a coherently pre pared atomic gas exhibit dissipation-free nonlinear coupling of an unpreced ented strength. This enables a single-photon pulse to coherently control or manipulate the quantum state of the other. Processes of this kind result i n generation of entangled states elf radiation field and open up new prospe ctives for quantum information processing.