Extending loophole-free nonlocal correlations to arbitrarily large distances

TL;DR

This paper demonstrates that loophole-free nonlocal correlations in quantum experiments can be extended over arbitrarily large distances by exploiting specific measurement strategies and device properties, overcoming previous distance limitations.

Contribution

It introduces a method to extend loophole-free nonlocal correlations to arbitrary distances using optimal strategies and analytical tradeoffs, surpassing previous experimental constraints.

Findings

Loophole-free nonlocal correlations can be extended over unlimited distances.

Perfect devices near the source enable long-distance nonlocality with minimal detection efficiency.

An analytical tradeoff allows relaxing requirements away from the source based on near-source nonlocality.

Abstract

One of the most striking features of quantum theory is that it allows distant observers to share correlations that resist local hidden variable (classical) explanations, a phenomenon referred to as Bell nonlocality. Besides their foundational relevance, the nonlocal correlations enable distant observers to accomplish classically inconceivable information processing and cryptographic feats such as unconditionally secure device-independent key distribution schemes. However, the distances over which nonlocal correlations can be realized in state-of-the-art Bell experiments remain severely limited owing to the high threshold efficiencies of the detectors and the fragility of the nonlocal correlations to experimental noise. Instead of looking for quantum strategies with marginally lower threshold requirements, we exploit the properties of loophole-free nonlocal correlations, which are experimentally attainable today, albeit at short distances, to extend them over arbitrarily large distances. Specifically, we consider Bell experiments wherein the spatially separated parties randomly choose the location of their measurement devices in addition to their measurement settings. We demonstrate that when devices close to the source are perfect and witness extremal loophole-free nonlocal correlations, such correlations can be extended to devices placed arbitrarily far from the source, with almost-zero detection efficiency and visibility. To accommodate imperfections close to the source, we demonstrate a specific analytical tradeoff: the higher the loophole-free nonlocality close to the source, the lower the threshold requirements away from the source. We utilize this analytical tradeoff paired with optimal quantum strategies to estimate the critical requirements of a measurement device placed away from the source and formulate a versatile numerical method applicable to generic network scenarios.