Entanglement-Enhanced Optical Interferometry in a Quantum Network

At a low light level, the sensitivity of optical long-baseline interferometry—central to astronomical and remote imaging—is limited by the difficulty of coherently combining weak optical modes without added loss or noise. We experimentally demonstrate an approach that enhances interferometric sensitivity in the low-photon regime by exploiting entanglement between spatially separated quantum nodes. In our implementation, quantum memories based on silicon-vacancy centers in diamond nanocavities are first entangled in an event-ready manner. An incoming signal photon then interacts locally with the memories; a photon-erasure step removes which-path information of the signal photon, while non-destructive photon heralding enabled by entanglement identifies successful joint phase-sensing events and boosts sensitivity. We perform differential phase measurements of weak thermal light over fiber baselines up to 1.55 km and a 6 m line-of-sight separation between nodes. The results demonstrate improved sensitivity over local detection, establishing a scalable route toward quantum-enhanced telescopy and long-baseline quantum interferometry with quantum memories.