When can Light GO Faster than Light? the Tunneling Time and its SubFemtosecond Measurement via Quantum Interference.
Abstract
The quantum properties of light are reviewed, in particular as they are illuminated in twophoton interference effects. Special attention is paid to EinsteinPodolsky Rosenlike correlations and to the effects of "whichpath" (welcher Weg) information on interference. Theoretical and experimental results are presented demonstrating that these effects are useful for highprecision measurements of singlephoton propagation times, and make these measurements largely insensitive to groupvelocity dispersion. We confirm that single photons in glass travel at the group velocity, an example of 'waveparticle unity.' The longstanding controversy over the duration of quantummechanical tunneling is briefly described, and an analogy is drawn with evanescent wave phenomena in optics. The relationship between one and twodimensional tunneling times is discussed at some length, and certain general results relating transmission and reflection delays are derived. An experiment is presented in which a multilayer dielectric mirror, described as a "photonic bandgap" medium, serves as a tunnel barrier. The peaks of those singlephoton wave packets which tunnel through this mirror are observed to arrive sooner than those which travel instead through an equivalent length of air. It is explained that this does not violate relativistic causality. Interpretational issues are analyzed to some extent. Another proposed experiment is described, in which similar effects could be observed with 100% transmission; such a scheme might be useful for compensating propagation delays in optical transmissions, and should be faithful even at the singlephoton level. Finally, it is argued that there is a sensible way to define conditional probabilities in quantum theory, and that these probabilities describe physically measurable effects. They are related to Aharonov et al.'s concept of "weak measurements" and the Feynmanaveraging procedure pioneered by Sokolovski et al. They allow one to discuss the history of a particle which has tunnelled, as distinct from that of those members of the initial ensemble which were reflected. I attempt to show that this approach unifies many of the previous discussions of tunneling times, in particular elucidating the meaning of imaginary momenta and complex times.
 Publication:

Ph.D. Thesis
 Pub Date:
 1994
 Bibcode:
 1994PhDT.......314S
 Keywords:

 Physics: General; Physics: Optics