(Phys.org)—Physicists have experimentally demonstrated an optical system based on an unconventional class of quantum mechanical systems that could lead to the development of new quantum optical devices. The system is called a “PT-symmetric quantum walk,” since it consists of single photons that occupy a superposition of states, called quantum walks, that obey parity-time (PT) symmetry—the property in which a system’s coordinates in space and time can have their signs reversed without inherently changing the system.