Quantum spectrum of black holes
  The black hole as the thermodynamical system in equilibrium possesses the periodicity of motion in imaginary time, that allows us to formulate the quasi-classical rule of quantization. The rule yields the equidistant spectrum for the entropy of non-rotating black holes as well as for the appropriately scaled entropy in the case of rotation. We clarify and discuss a role of quasi-normal modes. 
V.V.Kiselev