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Quantum Information

  1. Oct 10, 2013 #1
    A big hello to all physicist, chemist, engineers and anybody reading this thread,

    Back in the old days, information that used to be transmitted was analogue. Later it became numerical improving immunity vs noise; However I read a couple of days ago articles about the possibility of information being transmitted by speeds exceeding c, and about >10000c, and this being possible due to the fact that there is no particle used in the transmission medium, as it's using quantum entanglement of particles, and classical bits, are substituted by qubits (quantum bits).

    Does any person has an in-depth explanation on how does it work (Can give examples, forumlaes w/e) or can provide links that can clarify this?

    I found this link: http://arstechnica.com/science/2010/01/a-tale-of-two-qubits-how-quantum-computers-work/

    Not sure if it's good enough though.

    I need your help and opining on this matter, thanks :smile:
  2. jcsd
  3. Oct 10, 2013 #2


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    There is NO information transferred via entanglement and there is NO mechanism for transmitting information faster than c. Any statements to the contrary are crackpottery.
  4. Oct 10, 2013 #3
    How about quantum computers? Any clue about it? Or just another figment of the imagination of people?
  5. Oct 10, 2013 #4

    There is and can be no useful information transfer. This is certain.

    Quantum computers are supposed one day to work many orders of magnitude more efficiently than ordinary computers due to the multiplication of available hardware in the quantum realm where under certain circumstances, the term 'physical'(that from which gates are made of) carries a whole new meaning(basically imagine a 4 core processor becoming a 400 000 core processor at the flip of a switch).
    Last edited: Oct 10, 2013
  6. Oct 10, 2013 #5
    I don't have a solid background in quantum mechanics to be able to discuss with the more averted fellows in this domain, but what kinda mislead me was this, perhaps I misunderstood it:

    "Alice could send bits to Bob in the following way:
    If Alice wishes to transmit a '0', she measures the spin of her electron in the z direction, collapsing Bob's state to either |z+>B or |z->B. If Alice wishes to transmit a '1', she measures the spin of her electron in the x direction, collapsing Bob's state to either |x+>B or |x->B. Bob creates many copies of his electron's state, and measures the spin of each copy in the z direction. If Alice transmitted a '0', all his measurements will produce the same result; otherwise, his measurements will be split evenly between +1/2 and -1/2. This would allow Alice and Bob to communicate across space-like separations."
  7. Oct 10, 2013 #6


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    I'm not an expert either but I absolutely guarantee you that it won't work and I'm sure one of our more knowledgeable members will give you chapter and verse on specifically why your particular scenario doesn't work. There is NO information sent by entanglement. It requires slower than light communications to compare states to even see that the particles WERE entangled.

    Quantum computers have no relevance to your original question, since they do not do anything FTL.
  8. Oct 10, 2013 #7


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    All Bob ever sees, regardless of what Alice does, is a random sequence of + and -. This is regardless of whether Bob measures along x or along z. I think you know that a random sequence produces no information.

    Keep in mind that Alice cannot force Bob's particle into a particular state. She can only take a reading that will tell her something about what Bob's outcome will be.

    No useful FTL information transfer. :smile:
  9. Oct 10, 2013 #8
    It's absolutely right that the combination of entanglement and quantum measurement does not allow for FTL signaling. However, this result strongly relies on the seemingly fundamental randomness that we observe in the quantum measurement process, and precisely there also lies one of the greatest mysteries of modern physics.

    So I would be careful with absolute statements about the impossibility of FTL transfer until we have gained a more in-depth understanding of the quantum measurement process. I leave it to your imagination how new insight might or might not change something fundamental enough to uncover new physics. That said, I wouldn't bet my life on the no-signaling theorem.


  10. Oct 10, 2013 #9
    Quantum states can't be copied.

    There is no faster than light information transfer, ever. Only the correlation of information may be transmitted faster than light. But it doesn't allow communication. It even does not allow any influence on the system state.

    A researcher examining Bob's system will not see anything unusual, no matter what Alice is doing. He will not see any change in Bob's system behavior, and will not even be able to see if Alice is doing anything. A researcher can not determine if there exist any particles entangled with Bob's somewhere in the Universe and much less can he know what some distant entity is doing with them.

    Only when the researcher compares Bob's and Alice's systems, then he can observe correlations in some of their properties and deduce they were entangled. But such observation requires slower than light communication anyway.
  11. Oct 10, 2013 #10
    Indeed no FTL communication can work.

    I take the following as where you're coming from: you aren't worried about the particular result you get when measuring in either x, y or z axis, you are communicating the information by measuring in either one of those three axis. However, Bob won't know what direction Alice measured in. So if she measured in the x axis, Bob may measure in the y axis. From his result, he won't know if Alice measured in the x axis or not.
  12. Oct 11, 2013 #11


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    The only thing you can know about the system is that when Bob measures his particle it will correlate with Alice's result. As said previously, Alice measuring does not "cause" anything.

    Say she measures in the X axis. She will get + or -. If Bob measures in X he will get the opposite (+ or -). Say she measures in Y, she gets + or -. Bob measures in X again (because he always measures in the same axis since he has no information) and he gets + or -. So in each case, Bob get + or - regardless of what Alice did. For some reason (called entanglement) the results will always correlate if reviewed together afterwards, but they are random if considered alone.

    Quantum computers are in no way tied to FTL, but rather are about algorithms that can exploit multiple entangled particles having superpositions of state.
  13. Oct 15, 2013 #12
    As everyone else has already said, entanglement creates an apparent FTL coordination of activity, but this can never be used for FTL communication.
    Of course, computers based on quantum data processing (QDP) are currently very primitive. But even in principle, QDP would not be faster that conventional DP for all applications. In some cases, massively paralleled operations can be encoded as quantum superposition - resulting in QDP solutions where conventional techniques would consume more time and energy resources that are available in our universe. Most famously, factoring very large composite numbers could, in principle, by done by QDP - although a device capable of outperforming classical processors at this task has yet to be built.

    QDP does not depend on FTL information transmission, only on complex quantum superpositioning.
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