Negative phase time for Scattering at Quantum Wells: A Microwave Analogy   Experiment
  If a quantum mechanical particle is scattered by a potential well, the wave function of the particle can propagate with negative phase time. Due to the analogy of the Schr\"odinger and the Helmholtz equation this phenomenon is expected to be observable for electromagnetic wave propagation. Experimental data of electromagnetic wells realized by wave guides filled with different dielectrics confirm this conjecture now. 
R.-M. Vetter, A. Haibel, G. Nimtz