Comment on "Symmetry Breaking and Laser-Absorber Modes in Optical   Scattering Systems"
  We show that PT-symmetry may exist only for isolated frequencies in optical systems. Therefore, PT-symmetry breaking transitions as the frequency is tuned up, discussed in Y. D. Chong, L. Ge, and A. D. Stone, Phys. Rev. Lett. 106, 093902 (2011), cannot be observed in measurements of the spectrum of scattered intensity. 
A. A. Zyablovsky, A. P. Vinogradov, A. V. Dorofeenko, A. A. Pukhov,   and A. A. Lisyansky
cond-mat.mtrl-sci