The Bell inequality tells us (in effect) that if two photons (for example) were entangled when emitted, then we have a 50% chance of being able to detect that they were no longer entangled when they were received. To rephrase that, if they are not entangled when they are received, they still have a 50% chance of having that orientation that they would have had if they had still been entangled.(adsbygoogle = window.adsbygoogle || []).push({});

If one has (again, say) 30 pairs of entangled photons, one has only a 1 in a billion chance of not detecting if all 30 have been disentangled, but reports (such as those out today) suggest one can achieve probability 1 of detecting. Is that possible? If so how?

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# B How does quantum entanglement protect a key?

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