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

• DOI: 10.1038/nphoton.2016.144
• Publication Date: 2016-08-08T18:59:18Z
• Authors: Jian-Shun Tang; Yi-Tao Wang; Shang Yu; De-Yong He; Jin-Shi Xu; Bi-Heng Liu; Geng Chen; Yong-Nan Sun; Kai Sun; Yong-Jian Han; Chuan-Feng Li; Guang-Can Guo
• Keywords: Parity (physics); Signalling

The violation of the no-signalling principle — information can be transmitted faster than light — is experimentally investigated using entangled photons. It can be simulated when the parity–time symmetrically evolved subspace is solely considered. The experimental progress achieved in parity–time ( ) symmetry in classical optics1,2,3,4,5,6,7,8,9,10,11,12,13,14 is the most important accomplishment in the past decade15 and stimulates many new applications, such as unidirectional light transport5,6,7,8 and single-mode lasers12,13. However, in the quantum regime, some controversial effects are proposed for -symmetric theory16,17,18,19, for example, the potential violation of the no-signalling principle19. It is therefore important to understand whether -symmetric theory is consistent with well-established principles. Here, we experimentally study this no-signalling problem related to the -symmetric theory using two space-like separated entangled photons, with one of them passing through a post-selected quantum gate, which effectively simulates a -symmetric evolution. Our results suggest that the superluminal information transmission can be simulated when the successfully -symmetrically evolved subspace is solely considered. However, considering this subspace is only a part of the full Hermitian system, additional information regarding whether the -symmetric evolution is successful is necessary, which transmits to the receiver at maximally light speed, maintaining the no-signalling principle.