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 Rb-87 atoms. A resonant probe pulse with an energy per unit area of one photon per,lambda(2)/2 pi 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 lambda(2)/2 pi, 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. (c) 2005 Optical Society of America.