One of the biggest problems of modern physics is the enigmatic relationship between the two most successful fundamental theories of physical science: gravity described by general relativity and quantum mechanics. Despite extensive research, a satisfactory theory of quantum gravity remains elusive, with many unanswered questions requiring experimental validation. Recently, two proposals indicate a path to test, through tabletop experiments, a central question in quantum gravity research: has gravity a quantum behavior?

The image on the cover for Advanced Photonics Nexus Volume 3 Issue 3 pictorially illustrates a photonic quantum simulator able to reproduce the analog of the core principle behind these proposals, i.e., mediated entanglement. The paths of single photons, interacting in a photonic gate, are able to mediate entanglement between the polarizations of the photons which never directly interact. This means that the paths, simulating the gravitational field, have quantum properties.

The image is based on original research presented in the article “Photonic implementation of quantum gravity simulator” by Emanuele Polino, Beatrice Polacchi, Davide Poderini, Iris Agresti, Gonzalo Carvacho, Fabio Sciarrino, Andrea Di Biagio, Carlo Rovelli, and Marios Christodoulou (doi 10.1117/1.APN.3.3.036011)