  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 $\pi$ onto another light field. Here we
experimentally demonstrate the generation of such a $\pi$ phase 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
electromagnetically induced transparency creates a phase shift for a second
light pulse which propagates through the medium. Postselected on the detection
of a retrieved photon from the first pulse, we measure a $\pi$ phase shift of
the second pulse. This demonstrates a crucial step toward a photon-photon gate
and offers a variety of applications in the field of quantum information
processing.
