Fig. 3: Visualization of a version of Feynman’s experiment.

Two spherical mass distributions (1 and 2) of radius R are placed in quantum superpositions at two locations as N00N states, with blue and red denoting the components separated by Δx. After gravitationally interacting for a short time, the paths are recombined and entanglement is sought^5,6. Although Stern–Gerlach interferometry with internal spins is illustrated⁵, alternative set-ups, such as parallel Mach–Zehnders, are also possible⁶. Here, Δx is depicted larger than the minimum separation d_RL, but a general configuration can be implemented, including Δx ≪ d_RL.