One of the strangenesses concerning the behaviour of particles crossing a tunnel barrier is that one : crossing the tunnel barrier at superluminal group velocity is theoretically possible (see Wigner time and Hartman effect).
This has been experimentally confirmed by experiments conducted by R. Chiao with the participation of P. Kwait and A. Steinberg at Berkeley University (1993-1995). These experiments were devoted to the measurement of the tunnelling time of photons at a wavelength of 702 nm.
These measurement were achieved by comparison with the travelling time of "EPR twins" photons. These EPR twins were travelling the same optical path length, but in vacuum. They were created by Parametric Down Conversion, hence at the same time than their "EPR twins" crossing the tunnel barrier. The comparison of arrival times rested on the use of a Hong, Ou and Mandel interferometer.
The tunnel barrier consisted of a dielectric mirror with eleven quarter-wavelength layers of alternately high index material (titanium oxide with n = 2.22) and low index material (fused silica with n = 1.45). The total thickness of the eleven layers was 1.1 μm. This implied an in vacuo traversal time across the structure of 3.6 fs.
The data in Fig. 3(a) implies that after traversing the tunnel barrier, the peak of a photon wave packet arrived 1.47 ± 0.21 fs earlier than it would had it traversed only vacuum.
Tunneling Times and Superluminality: a Tutorial, Raymond Y. Chiao