Low-light-level photon switching by quantum interference

We report an experimental demonstration of low-light-level photon switching by quantum interference in laser-cooled Rb87 atoms. A resonant probe pulse with an energy per unit area of one photon per ?2?2? propagates through the optically thick atoms. Its energy transmittance is greater than 63%, or a loss of less than e?1, because of the effect of electromagnetically induced transparency. In the presence of a switching pulse with an energy per unit area of 1.4 photons per ?2?2?, the energy transmittance of the same probe pulse becomes less than 37%, or e?1. This substantial reduction of probe transmittance caused by switching photons may lead to potential applications in single-photon-level nonlinear optics and manipulation of quantum information.