Virtual single-photon transition interrupted : Time-gated optical gain and loss

This work was supported by the National Center of Competence in Research Molecular Ultrafast Science and Technology (NCCR MUST), research instrument of the Swiss National Science Foundation. P.R. acknowledges a Juan de la Cierva Contract Grant from MICINN, and the COST Action CM0702. We thank H. R. Reiss and M. Lucchini for fruitful discussions