  All-optical switching is a technique in which a gate light pulse changes the
transmission of a target light pulse without the detour via electronic signal
processing. We take this to the quantum regime, where the incoming gate light
pulse contains only one photon on average. The gate pulse is stored as a
Rydberg excitation in an ultracold atomic gas using electromagnetically induced
transparency. Rydberg blockade suppresses the transmission of the subsequent
target pulse. Finally, the stored gate photon can be retrieved. A retrieved
photon heralds successful storage. The corresponding postselected subensemble
shows an extinction of 0.05. The single-photon switch offers many interesting
perspectives ranging from quantum communication to quantum information
processing.
