ATOMIC COHERENCE EFFECTS WHICH PRODUCE SUPERLUMINAL (BUT CAUSAL PROPAGATION OF WAVEPACKETS

Analytic, limited-bandwidth signals, e.g. Gaussian wavepackets, whose frequencies lie in a transparent spectral window far below the resonan ce of an amplifying atomic medium, can propagate with phase, group, en ergy and 'signal' velocities (as defined by Sommerfeld and Brillouin) all exceeding the vacuum speed of light c. This 'superluminal' propaga tion may occur without significant distortion or gain. The front veloc ity, however, is not superluminal, and therefore Einstein causality is not violated. Applications will be discussed, including the possibili ty of using superluminal atomic coherence in condensed matter (e.g. ma croscopic mass currents in superfluid helium which arise from vortex m otions) for efficient gravitational antennae.