First experimental evidence for chaos-assisted tunneling in a microwave annular billiard

We report on first experimental signatures for chaos-assisted tunneling in a two-dimensional annular billiard. Measurements of microwave spectra from a superconducting cavity with high frequency resolution are combined with e lectromagnetic field distributions experimentally determined from a normal conducting twin cavity with high spatial resolution to resolve eigenmodes w ith properly identified quantum numbers. Distributions of quasidoublet spli ttings serve as basic observables for the tunneling between whispering gall ery-type modes localized to congruent, but distinct tori which are coupled weakly to irregular eigenstates associated with the chaotic region in phase space.