On the quantum spectrum of isochronous potentials
  In this paper, the quantum spectrum of isochronous potentials is investigated. Given that the frequency of the classical motion in such potentials is energy-independent, it is natural to expect their quantum spectra to be equispaced. However, as it has already been shown in some specific examples, this property is not always true. To gain some general insight into this problem, a WKB analysis of the spectrum, valid for any analytic potential, is performed and the first semiclassical corrections to its regular spacing are calculated. We illustrate the results on the two-parameter family of isochronous potentials derived in [1], which includes the harmonic oscillator, the asymmetric parabolic well, the radial harmonic oscillator and Urabe's potential as special limiting cases. In addition, some new analytical expressions for families of isochronous potentials and their corresponding spectra are derived by means of the above-mentioned method. 
J. Dorignac