Abstract
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 ~100 aJ, equivalent to ~400 photons, was observed in a four-level system of cold Rb87 atoms. © 2011 American Physical Society.