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One of the most striking features of quantum theory is that it allows distantobservers to share correlations that resist local hidden variable (classical)explanations, a phenomenon referred to as Bell nonlocality. Besides theirfoundational relevance, the nonlocal correlations enable distant observers toaccomplish classically inconceivable information processing and cryptographicfeats such as unconditionally secure device-independent key distributionschemes. However, the distances over which nonlocal correlations can berealized in state-of-the-art Bell experiments remain severely limited owing tothe high threshold efficiencies of the detectors and the fragility of thenonlocal correlations to experimental noise. Instead of looking for quantumstrategies with marginally lower threshold requirements, we exploit theproperties of loophole-free nonlocal correlations, which are experimentallyattainable today, albeit at short distances, to extend them over arbitrarilylarge distances. Specifically, we consider Bell experiments wherein thespatially separated parties randomly choose the location of their measurementdevices in addition to their measurement settings. We demonstrate that whendevices close to the source are perfect and witness extremal loophole-freenonlocal correlations, such correlations can be extended to devices placedarbitrarily far from the source, with almost-zero detection efficiency andvisibility. To accommodate imperfections close to the source, we demonstrate aspecific analytical tradeoff: the higher the loophole-free nonlocality close tothe source, the lower the threshold requirements away from the source. Weutilize this analytical tradeoff paired with optimal quantum strategies toestimate the critical requirements of a measurement device placed away from thesource and formulate a versatile numerical method applicable to generic networkscenarios.

Extending loophole-free nonlocal correlations to arbitrarily large distances