QUANTUM OPTICAL STUDIES OF TUNNELING AND OTHER SUPERLUMINAL PHENOMENA

Experiments at Berkeley and elsewhere which show that the process of t unneling is apparently superluminal will be reviewed. Conflicting theo ries for the tunneling time will be compared with experiment. The tunn eling particle in the Berkeley experiments was the photon. The measure ment of the tunneling time utilized a two-photon light source (spontan eous parametric down-conversion), a Hong-Ou-Mandel interferometer, and a coincidence counter of photon pairs. The tunnel barrier consisted o f a photonic-bandgap medium excited at midgap. We find that the peak o f a tunneling single-photon wave packet appeared on the far side of th e barrier 1.47 +/- 0.21 fs earlier than the peak of a wave packet whic h traveled an equal distance in sis, Group velocities which exceed c c an also occur in transparent media with inverted atomic populations. T achyon-like excitations, i.e., collective normal modes of atoms couple d to electromagnetic radiation which possess tachyon-like dispersion r elations, occur in these media. However, relativistic causality is not violated by any of these superluminal phenomena.