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The EPR Crypto-system

  1. Jan 12, 2005 #1
    In the Oct 1992 issue of Scientific American, the article "Quantum
    Cryptography", by Charles Bennet, Gilles Brassard and Artur Ekert
    mainlines experimental work on a very powerful cryptosystem that has
    been devised that is essentially impossible under QM rules:

    "The EPR effect occurs when a spherically symmetric atom emits two photons in opposite directions toward two observers, Alice and Bob. The two photons are produced in an initial state of undefined polarization. But because of the symmetry of the initial state, the polarizations of the photons, when measured, must have opposite values, provided that the measurements are of the same type. For example, if Alice and Bob both measure rectilinear polarizations, they are each equally likely to record either a 0 (horizontal polarization) or a 1 (vertical), but if Alice obtains a 0, Bob will certainly obtain a 1, and vice versa.

    The unusual and important aspect of the EPR effect is that the polarization of both photons is determined as soon as, but not before, one of the photons is measured. This happens no matter how far apart the photons may be at the time. This "classical" explanation of the EPR effect is somewhat counterintuitive, and indeed all classical explanations of the EPR effect involve some implausible element, such as instantaneous action at a distance. Yet the mathematical formalism of quantum mechanics accounts for the EPR effect in a straightforward manner, and experiments have amply confirmed the existance of the phenomenon."

    The crypto-system of Bennet, Brassard and Ekert is now the basis of several commercial products, so the EPR experiment is being performed on a daily basis.

    All the best
    John B.
    Last edited: Jan 12, 2005
  2. jcsd
  3. Jan 12, 2005 #2


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    I believe you have read all of this backwards - which is very easy to do. The EPR setup described above is expected to operate the same way regardless of whether you are talking QM or classical reality.

    The situation changes when you postulate that the polarization exists independently of the measurement. I do not believe any crypto-system uses this assumption as its cornerstone. In fact, I doubt you will see many working EPR type setups used for sending coded messages, much less on a daily basis. :smile: They tend to be expensive and difficult to operate.
    Last edited: Jan 12, 2005
  4. Jan 13, 2005 #3


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    Note the contradiction:

    BTW, the second statement is the correct one.

  5. Jan 14, 2005 #4
    Impossible Under QM Rules

    The QM hypothesis is that both photons are in a state of superposition until one is measured. Once one is measured, then the other reacts and becomes the opposite polarization. This requires faster than light "spooky action at a distance." If "spooky action at a distance" was possible, then engineers would be able to use it to send signals. I am an electrical engineer and I am here to tell you that we cannot send signals using "spooky action at a distance."

    All the best
    John B.
  6. Jan 14, 2005 #5


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    That is incorrect. Spooky action at a distance cannot be used to send information. Entangled pairs could concievably be used as a sort of one-time-pad or for other kinds of synchronization.
    Let's say we start with Abe and Betty who want to use entangled pairs to communicate, and let's also say that they have an infinite supply of these pairs.
    So, Abe wants to send a signal to Betty, so he somehow manipulates his electron (or whatever) to get it to have spooky action at a distance with Betty's electron. Regarldess of what Abe did, Betty cannot, just by measuring her electron, identify whether Abe has manipulated his electron.
  7. Jan 14, 2005 #6


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    I'm also an electrical engineer, and I am here to tell you that you have no idea what you're talking about. Communication by quantum entanglement will ALWAYS require some form of additional classical channel. If you really understood quantum mechanics, you'd not make these kinds of mistakes. You're still stuck on the "paradox" of EPR, which makes you about a century late to the party.

    I am also here to kindly encourage you to either refrain from posting on subjects with which you are not well-acquainted, or to find yourself greener pastures which will tolerate the sort nonsense posts you've made here. It won't be tolerated much longer here.

    - Warren
    Last edited: Jan 14, 2005
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