Hydrogen atom in crossed electric and magnetic fields: Phase space   topology and torus quantization via periodic orbits
  A hierarchical ordering is demonstrated for the periodic orbits in a strongly coupled multidimensional Hamiltonian system, namely the hydrogen atom in crossed electric and magnetic fields. It mirrors the hierarchy of broken resonant tori and thereby allows one to characterize the periodic orbits by a set of winding numbers. With this knowledge, we construct the action variables as functions of the frequency ratios and carry out a semiclassical torus quantization. The semiclassical energy levels thus obtained agree well with exact quantum calculations. 
Stephan Gekle, J\"org Main, Thomas Bartsch, T. Uzer