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B How does quantum entanglement protect a key?

  1. Jul 11, 2017 #1
    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.

    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?
  2. jcsd
  3. Jul 11, 2017 #2


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    If you could give us an idea of what you are reading, a more concise answer to your question would be possible.
  4. Jul 11, 2017 #3
    My question was prompted by recent news, such as http://www.nature.com/news/china-s-...-on-way-to-ultrasecure-communications-1.22142, but does not depend on it.

    So more simply, can quantum entanglement be used to enable a key to be transmitted in the certain knowledge that the receiver can tell if it has already been read? If so, how? Or is it only possible to achieve arbitrarily high probability that the receiver can tell if the key has already been read?
  5. Jul 11, 2017 #4


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    Google for "quantum key distribution" - the Wikipedia article is pretty good.
  6. Jul 12, 2017 #5
    Thanks, it is indeed pretty good.

    I am (almost) confident that it confirms my understanding that quantum communicator can detect eavesdropping with probability approaching arbitrarily close to 1, but not with complete certainty. The difference is perhaps of no interest to most people, which is why even the Wiki article makes statements like "Second, any attempt at eavesdropping by Eve destroys these correlations in a way that Alice and Bob can detect." rather than "... almost certainly detect".

    I say I am 'almost' confident because I am aware that one can feel certain of something and yet be wrong. In the Wiki article it is not difficult to read it without realising that 'confident' means 'effectively confident'. I hoped that respected people here would be able to confirm my conclusion, even if they also felt it necessary to point out I am nit picking. I hoped more fervently that if I am mistaken, someone here could point out why.
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