Curator's Take
This article shows that the way measurements are arranged—rather than just the amount of entanglement—can be a decisive resource for demonstrating nonlocal correlations across quantum networks, a nuance that has been largely overlooked until now. By experimentally certifying bilocal and fully‑network nonlocality on IBM’s 156‑qubit superconducting processor, the work provides one of the first concrete demonstrations that programmable hardware can verify complex network‑level quantum behavior, complementing recent advances in entanglement‑swapping and quantum‑internet prototypes. The findings warn that even maximally entangled states may fail to reveal nonlocality if the joint measurement geometry is suboptimal, highlighting a new design dimension for future scalable quantum communication protocols.
— Mark Eatherly
Summary
Quantum networks can exhibit nonclassical correlations that cannot be explained by classical models with independent sources. While the role of entanglement is well understood, the impact of measurement design remains largely unexplored. Here we develop an operational framework for certifying network nonlocality in the bilocal Alice--Bob--Charlie network using ancilla-assisted meters to evaluate the nonlocal observables required for bilocal and fully network nonlocal (FNN) witnesses. The approach successfully reproduces both bilocal and FNN correlations in simulation. On the 156-qubit superconducting processor \textit{ibm\_kingston}, we observe bilocal nonlocality with $\mathcal{S}_{\rm BLHV}=1.067(6)>1$ after readout-error mitigation, while the FNN witnesses reach $99\%$ and $96\%$ of their certification thresholds, implying the substantially stronger requirements for FNN certification. We further show that Bob's joint measurement determines the accessible level of network nonlocality: bilocal and FNN certification are optimized by different measurement settings, while both violations can disappear even for maximally entangled states. These results identify measurement geometry as an independent resource for network nonlocality and provide a practical route toward its certification on programmable quantum processors.