Comment on: Direct space-time observation of pulse tunneling in an electromagnetic band gap, S.Doiron,A.Hache,H.Winful

TL;DR

This paper critiques a previous study, clarifying that the observed superluminal effects are related to resonator dispersion rather than true quantum tunneling, emphasizing the distinction between evanescent wave phenomena and tunneling.

Contribution

It clarifies the misinterpretation of superluminal effects as tunneling in the referenced work, emphasizing the correct physical mechanisms involved.

Findings

Superluminal group velocity observed is due to anomalous dispersion, not tunneling.

Tunneling involves evanescent modes with imaginary wave numbers, distinct from the studied resonator.

Signals with evanescent components can appear superluminal without violating causality.

Abstract

The title of this article is misleading. The authors have investigated a resonator but not a tunneling barrier see also Refs.\cite{Winful2} The measured superluminal group velocity and discussed is that studied on a Lorentz-Lorenz oscillator by Sommerfeld and Brillouin a hundred years ago \cite{Brillouin}. It is similar to the faster than light experiment by Wang et al. based also on anomalous dispersion with a complex refractive index of a resonator \cite{Wang}. Tunneling, however, is understood and performed by electromagnetic evanescent modes or by tunneling solutions of the Schr\"odinger equation, which have purely imaginary wave numbers. The latter includes a purely imaginary refractive index. Signals with purely evanescent frequency components can travel at a superluminal velocity \cite{NimtzH,Nimtz1}. Inside the barrier tunneling proceeds even instantaneously, i.e.by a process described by virtual photons \cite{Stahlhofen}.