  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.
