PT symmetry and spontaneous symmetry breaking in a microwave billiard
  We demonstrate the presence of parity-time (PT) symmetry for the non-Hermitian two-state Hamiltonian of a dissipative microwave billiard in the vicinity of an exceptional point (EP). The shape of the billiard depends on two parameters. The Hamiltonian is determined from the measured resonance spectrum on a fine grid in the parameter plane. After applying a purely imaginary diagonal shift to the Hamiltonian, its eigenvalues are either real or complex conjugate on a curve, which passes through the EP. An appropriate basis choice reveals its PT symmetry. Spontaneous symmetry breaking occurs at the EP. 
S. Bittner, B. Dietz, U. Guenther, H. L. Harney, M. Miski-Oglu, A.   Richter, F. Schaefer
quant-ph math-ph math.MP