  The response of matter to an optical excitation consists essentially of
absorption and emission. Traditional spectroscopy accesses the
frequency-resolved and time-integrated response, while the temporal evolution
stays concealed. However, we will demonstrate here that the temporal evolution
of a virtual single-photon transition can be mapped out by a second pulsed
electromagnetic field. The resulting optical signal shows previously unexpected
optical gain and loss, which can be gated and controlled via the relative delay
of the electromagnetic fields. The model presented here can be applied to any
system that assumes a two-level character through near-resonant optical dipole
excitation, whether they are of atomic, molecular or even solid-state nature.
These theoretical observations are in excellent qualitative agreement with our
transient absorption spectroscopy study in helium. The presented results can
act as starting point for a new scheme for creating optical gain, which is a
prerequisite for the operation of lasers. It may be possible to open the doors
to spectral regions, which were difficult to access until now, e.g. in the
extreme ultraviolet.
