Exceptional points in bichromatic Wannier-Stark systems
  The resonance spectrum of a tilted periodic quantum system for a bichromatic periodic potential is investigated. For such a bichromatic Wannier-Stark system exceptional points, degeneracies of the spectrum, can be localized in parameter space by means of an efficient method for computing resonances. Berry phases and Petermann factors are analyzed. Finally the influence of a nonlinearity of the Gross-Pitaevskii type on the resonance crossing scenario is briefly discussed. 
C Elsen, K Rapedius, D Witthaut and H J Korsch
quant-ph