  Photons are excellent information carriers but normally pass through each
other without consequence. Engineered interactions between photons would enable
applications from quantum information processing to simulation of condensed
matter systems. Using an ensemble of cold atoms strongly coupled to an optical
cavity, we demonstrate experimentally that the transmission of light through a
medium may be controlled with few photons and even by the electromagnetic
vacuum field. The vacuum induces a group delay of 25 ns on the input optical
pulse, corresponding to a light velocity of 1600 m/s, and a transparency of 40%
that increases to 80% when the resonator is filled with 10 photons. This
strongly nonlinear effect provides prospects for advanced quantum devices such
as photon-number-state filters.
