Experimental investigation of the no-signalling principle in parity-time symmetric theory using an open quantum system

The experimental progress achieved in parity-time (PT) symmetry in classical optics(1-14) is the most important accomplishment in the past decade(15) and stimulates many new applications, such as unidirectional light transport(5-8) and single-mode lasers(12,13). However, in the quantum regime, some controversial effects are proposed for PT-symmetric theory(16-19), for example, the potential violation of the no-signalling principle(19). It is therefore important to understand whether PT-symmetric theory is consistent with well-established principles. Here, we experimentally study this no-signalling problem related to the PT-symmetric theory using two space-like separated entangled photons, with one of them passing through a post-selected quantum gate, which effectively simulates a PT-symmetric evolution. Our results suggest that the superluminal information transmission can be simulated when the successfully PT-symmetrically evolved subspace is solely considered. However, considering this subspace is only a part of the full Hermitian system, additional information regarding whether the PT-symmetric evolution is successful is necessary, which transmits to the receiver at maximally light speed, maintaining the no-signalling principle.