Experimental few-copy multi-particle entanglement detection

Quantum technologies lead to a variety of applications that outperform their classical counterparts. In order to build a quantum device it must be verified that it operates below some error threshold. Recently, because of technological developments which allow for the experimental realization of quantum states with increasing complexity, these tasks must be applied to large multi-qubit states. However, due to the exponentially-increasing system size, tasks like quantum entanglement verification become hard to carry out in such cases. Here we develop a generic framework to translate any entanglement witness into a resource-efficient probabilistic scheme. We show that the confidence to detect entanglement grows exponentially with the number of individual detection events. To benchmark our findings, we experimentally verify the presence of entanglement in a photonic six-qubit cluster state generated using three single-photon sources operating at telecommunication wavelengths. We find that its presence can be certified with at least 99.74% confidence by detecting 20 copies of the quantum state. Additionally, we show that genuine six-qubit entanglement is verified with at least 99% confidence by using 112 copies of the state. Our protocol can be carried out with a remarkably low number of copies, making it a practical and applicable method to verify large-scale quantum devices.