Heralded quantum entanglement between two crystals

Quantum networks must have the crucial ability to entangle quantum nodes(1). A prominent example is the quantum repeater(2-4), which allows the distance barrier of direct transmission of single photons to be overcome, provided remote quantum memories can be entangled in a heralded fashion. Here, we report the observation of heralded entanglement between two ensembles of rare-earth ions doped into separate crystals. A heralded single photon is sent through a 50/50 beamsplitter, creating a single-photon entangled state delocalized between two spatial modes. The quantum state of each mode is subsequently mapped onto a crystal, leading to an entangled state consisting of a single collective excitation delocalized between two crystals. This entanglement is revealed by mapping it back to optical modes and by estimating the concurrence of the retrieved light state(5). Our results highlight the potential of crystals doped with rare-earth ions for entangled quantum nodes and bring quantum networks based on solid-state resources one step closer.