Experimental random-party entanglement distillation via weak measurement

Maximally bipartite entangled state |Ψ^{+}〉, also known as the Einstein-Podolsky-Rosen pair, is the unit resource of entanglement and the key for quantum information processing. An important problem is that how many maximally bipartite entangled states could be distilled from a multipartite entangled state shared among a quantum network. Here, we focus on the distillation of |Ψ^{+}〉 from a single copy of the three-qubit W state. An interesting phenomenon in this case is that the random entanglement distillation between two unspecified parties can yield a strictly higher distillation rate (the average number of |Ψ^{+}〉 distilled from each W state) than the case between two specified parties. In this work, we develop a distillation protocol by introducing weak measurements that do not destroy the global entanglement. We find that the distillation rate can be significantly enhanced with only a few rounds by performing an extra distillation procedure between two specified parties at the final step. Experimentally, we prepare a three-photon W state in the polarization degree of freedom, and employ the path degree of freedom of each photon as probe qubits to realize the weak measurement. As a proof-of-principle demonstration, we show that the distillation rate is enhanced from 2/3, which is the theoretical limit of any distillation scheme between two specified parties, to 0.751±0.030 between two unspecified parties with only one distillation round.