Electromagnetically-induced-transparency-based cross-phase-modulation at attojoule levels

We report the experimental demonstration of electromagnetically-induced-transparency-based cross-phase-modulation at attojoule or, equivalently, few-hundred-photon levels. A phase shift of 0.005 rad of a probe pulse modulated by a signal pulse with an energy of similar to 100 aJ, equivalent to similar to 400 photons, was observed in a four-level system of cold Rb-87 atoms.