Optical π phase shift created with a single-photon pulse

A deterministic photon-photon quantum-logic gate is a long-standing goal. Building such a gate becomes possible if a light pulse containing only one photon imprints a phase shift of \pionto another light field. Here we experimentally demonstrate the generation of such a \piphase shift with a single-photon pulse. A first light pulse containing less than one photon on average is stored in an atomic gas. Rydberg blockade combined with...