Time from quantum entanglement: An experimental illustration

In previous years several theoretical papers discussed if time can be an emergent property deriving from quantum correlations. Here, to provide insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters' mechanism of "static" time [D. N. Page and W. K. Wootters, Phys. Rev. D 27, 2885 (1983), 10.1103/PhysRevD.27.2885], and Gambini et al. for subsequent refinements [R. Gambini et al., Phys. Rev. D 79, 041501(R) (2009), 10.1103/PhysRevD.79.041501]. A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: An "internal" observer that becomes correlated with the clock photon sees the other system evolve, while an "external" observer that only observes global properties of the two photons can prove it is static.