Electromagnetic fields dressed by inverted two-level atoms become tachyonlike excitations with group velocities which are faster than $c$, infinite, or negative. Such excitations describe the stable modes of the medium when it is weakly probed off resonance. The launching of these tachyonlike excitations is discussed, along with a proposed experiment to observe them. Their existence does not violate Einstein causality.

• Received 12 March 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.1254