Experimental studies of Chaos and Localization in Quantum Wavefunctions

Wavefunctions in chaotic and disordered quantum billiards are studied experimentally using thin microwave cavities. The chaotic wavefunctions display universal density distributions and density auto-correlations in agreement with expressions derived fro m a 0-D nonlinear $\sigma $-model of supersymmetry, which coincides with Random Matrix Theory. In contrast, disordered wavefunctions show deviations from this universal behavior due to Anderson localization. A systematic behavior of the distribution function is studied as a function of the localization length, and can be understood in the framework of a 1-D version of the nonlinear $\sigma $-model.