MEASUREMENT OF THE SINGLE-PHOTON TUNNELING TIME

Using a two-photon interferometer, we have measured the time delay for a photon to tunnel across a barrier consisting of a 1.1-mum-thick 1D photonic band-gap material. The peak of the photon wave packet appears on the far side of the barrier 1.47 +/- 0.21 fs earlier than it would if it were to travel at the vacuum speed of light c. Although the app arent tunneling velocity (1.7 +/- 0.2)c is superluminal, this is not a genuine signal velocity, and Einstein causality is not violated. The measured tunneling time is consistent with the group delay (''phase ti me''), but not with the semiclassical time.