Experimental Study of Active LRC Circuits with PT-Symmetries
  Mutually coupled modes of a pair of active LRC circuits, one with amplification and another with an equivalent amount of attenuation, provide an experimental realization of a wide class of systems where gain/loss mechanisms break the Hermiticity while preserving parity-time PT symmetry. For a value PT of the gain/loss strength parameter the eigen-frequencies undergo a spontaneous phase transition from real to complex values, while the normal modes coalesce acquiring a definite chirality. The consequences of the phase-transition in the spatiotemporal energy evolution are also presented. 
Joseph Schindler, Ang Li, Mei C. Zheng, F. M. Ellis, Tsampikos Kottos
cond-mat.other quant-ph