Using a two-photon interferometer, we have measured the time delay for a photon to tunnel across a barrier consisting of a 1.1-{μ}m-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 apparent 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 time''), but not with the semiclassical time.
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Steinberg et al. (1993) studied this question.
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