Efficient benchmarking of photonic cluster state sources

Certain single-photon sources are capable of emitting strings of entangled photons. In particular, photonic cluster state sources could enable measurement-based quantum computing and are therefore under intense investigation. While experiments are progressing, the issue of benchmarking comes to the fore.

The challenge is to quantify the useful long-range entanglement of a large photonic state using only the few-photon correlation measurements that are feasible given limited emission/detection efficiencies. We present a lower bound on localizable entanglement that requires only simple three-qubit correlation measurements. This method therefore enables direct demonstration of computationally powerful multi-photon entanglement with currently available experimental capabilities.