PT-Symmetric Photonics in the Absence of Gain or Loss

Our conventional understanding of physics is based on the fundamental notion of energy conservation that formally manifests itself in the real-valued energy spectra of Hermitian Hamiltonians. By contrast, so-called open systems can exchange energy with their environment, and, as subsystems of a larger whole, may exhibit non-Hermitian dynamics. Along these lines, non-Hermiticity established by local attenuation and coherent amplification has been firmly established on a variety of platforms. More recently, the related concept of parity-time symmetry has been associated with the wave-mechanical interplay of gain and loss in complex-valued potentials [1], [2]. In the context of quantum systems, however, the presence of actual gain inevitably gives rise to noise, which, in most cases, prevents a direct adaptation of “classical” non-Hermitian settings [3].