$1/f^{\alpha}$ Noise in Spectral Fluctuations of Quantum Systems
  The power law $1/f^{\alpha}$ in the power spectrum characterizes the fluctuating observables of many complex natural systems. Considering the energy levels of a quantum system as a discrete time series where the energy plays the role of time, the level fluctuations can be characterized by the power spectrum. Using a family of quantum billiards, we analyze the order to chaos transition in terms of this power spectrum. A power law $1/f^{\alpha}$ is found at all the transition stages, and it is shown that the exponent $\alpha$ is related to the chaotic component of the classical phase space of the quantum system. 
J.M.G. Gomez, A. Relano, J. Retamosa, E. Faleiro, L. Salasnich, M.   Vranicar, M. Robnik