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I Question about Qubits

  1. Dec 12, 2016 #1


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    Hello everyone!

    I'm having trouble understanding a specific aspect of qubits, maybe someone among you clever guys can help me.

    I understand that a qubit is in superposition, we can store information (a quantum property equivalent of true or false) in it. I also understand that reading that information leads to the collapse of the wave function, so subsequent read is not possible.
    1. set(true)
    2. read() = true
    3. read() = ?

    But can we use that qubit again? I mean can we restore the superposition and store another information in it after a read?
    1. set(true)
    2. read() = true
    3. set(true)
    4. read() = true?

    In other words is it possible to store information again on the same qubit, or that would be a totally independent qubit superposition with different wave function?

    Sorry if my question does not make sense, I'm not a physicist, just a programmer.
  2. jcsd
  3. Dec 12, 2016 #2
    Yes, but you will have erased the information from the 1st superposition, so no. You could maybe freeze the superposition as a third qbit number, but that would be very tricky in practice. You only get one (true or false), every time. Once it has been 're-entangled', with a magnet, such as in a spin liquid experiment, as far as I know, only has one 'random' spin choice, with a true (don't read the information) or false ( read the information and collapse the wave function). What AI computing is doing in Qbit spin memory is two tasks, 'not reading the information/spin' (keeping useful information) or 'reading the information' (getting rid of un-useful and bad information by collapsing the wave function). Or vice versa, depending on your program modeling.
    There is though, another snazzy technique that uses polarization logic gates and use 4 or 8 variations per q bit. put those pieces of information on a 'card' of lets say 144 qbits and by inter tangling those card numbers can create huge orders of processing, but the math is insanely difficult to program those number combinations into functions.

    Here is a Perimeter Institue lecture on the 4 atom amplitude technique. From one of the most prestigious quantum computing experts.
  4. Dec 12, 2016 #3


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    Thank you very much for your answer!
    So is it possible to keep (or re-establish) the entanglement after a read? I mean what if we have 2 qbits entangled at start? Would the 2nd superposition keep that spooky effect from the 1st superposition or it's erased along with the information?

    1. set(q1, true) (this would also set the spin of q2)
    2. read(q2) = true
    3. set(q1, true)
    4. read(q2) = true?

    Or would q1 and q2 became independent after step 2? Hope my question makes sense :-)

  5. Dec 12, 2016 #4
    The Born rule applies to measurement of Qbits, so in your first example in number three, where you have a question mark, that would read true as would any subsequent measurement.
  6. Dec 12, 2016 #5


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    Thank you!

    I've followed your lead, read about the role. That yield to another (hopefully final) question :-) Is it possible to have q1 and q2 entangled when the wave function is normalized? Or does normalization rule that out and give only one qbit per wave function?

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