= Hardy's paradox
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= Hardy paradox
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A two-setting nonlocality argument in which several zero-probability events forbid a further coincidence in any <deterministic local hidden-variable model>, but quantum theory assigns that coincidence positive <probability>. For binary indicators, suppose $C_+^RC_-^R=0$, $D_+^I=1\Rightarrow C_-^R=1$ and $D_-^I=1\Rightarrow C_+^R=1$. Any assignment with $D_+^ID_-^I=1$ would violate the first relation. A quantum implementation can nevertheless give such a coincidence. Real imperfect experiments require an inequality that accounts for their nonzero error probabilities. <Lucien Hardy> gave a two-particle inequality-free argument in 1992.<Quantum mechanics, local realistic theories, and Lorentz-invariant realistic theories by Lucien Hardy>[1992 abstract]{ref}
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