Entanglement-assisted non-local optical interferometry in a quantum network

The sensitivity of non-local optical measurements at low light intensities, such as those involved in long-baseline telescope arrays^1,2, is limited by fundamental quantum noise and photon losses³. Distributed quantum entanglement has been proposed as a route towards overcoming these limitations and accessing new regimes of non-local optical sensing^4-6. Here we demonstrate the use of entangled quantum memories in a quantum network of silicon-vacancy centres in diamond nanocavities^7-9 to experimentally perform such non-local phase measurements. Specifically, we combine the generation of event-ready remote quantum entanglement, photon mode erasure that hides the 'which-path' information of temporally and spatially separated incoming optical modes and non-local, non-destructive photon heralding enabled by remote entanglement to perform a proof-of-concept entanglement-assisted differential phase measurement of weak incident light between two spatially separate stations. Demonstrating successful operation of the remote phase sensing protocol with a fibre link baseline up to 1.55 km, our results provide an opportunity for a new class of quantum-enhanced optical imaging methods with potential applications ranging from long-baseline interferometry and astronomy to microscopy^10,11.