Noise-robust proofs of quantum network nonlocality

Quantum networks allow for novel forms of quantum nonlocality. By exploiting the combination of entangled states and entangled measurements, strong nonlocal correlations can be generated across the entire network. So far, all proofs of this effect are essentially restricted to the idealized case of pure entangled states and projective local measurements. Here we present noise-robust proofs of network quantum nonlocality, for a class of quantum distributions on the triangle network that are based on entangled states and entangled measurements. The key ingredient is a result of approximate rigidity for local distributions that satisfy the so-called “parity token counting'' property with high probability. Our methods can be applied to any type of noise. As illustrative examples, we consider quantum distributions obtained with imperfect sources and obtain a noise robustness up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&amp;#x223C;</mml:mo><mml:mn>80</mml:mn><mml:mi mathvariant="normal">&amp;#x0025;</mml:mi></mml:math> for dephasing noise and up to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&amp;#x223C;</mml:mo><mml:mn>0.5</mml:mn><mml:mi mathvariant="normal">&amp;#x0025;</mml:mi></mml:math> for white noise. Additionally, we prove that all distributions in the vicinity of some ideal quantum distribution are nonlocal, with a bound on the total-variation distance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>&amp;#x223C;</mml:mo><mml:mn>0.25</mml:mn><mml:mi mathvariant="normal">&amp;#x0025;</mml:mi></mml:math>. Our work opens interesting perspectives towards the practical implementation of quantum network nonlocality.