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Thread: Quantum computing: a qubit is both 0 and 1 at the same time!?

  1. #1 Quantum computing: a qubit is both 0 and 1 at the same time!? 
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    In classical computing, a byte is either 0 or 1. In quantum computing the equivalent to the byte, a qubit, is both 0 and 1 at the same time. I understand this is to do with the weird and wonderful nature of quantum mechanics whereby particles (?) can be in two places at once, or something like that.

    But if my byte is both 0 and 1, how is that useful in a computing sense? Surely I want it to be one or the other.

    Applied to real-world computing, this seems to suggest that the contents of my saved Word file is every possible permutation of characters, but of course I want it to contain just what I type into it. I'm struggling to correlate that with the idea that qubits do not have fixed value - rather, they have *every* value.

    I guess I'm making the mistake of applying human intuition, based on classical physics, to the very different world of quantum mechanics. But could anyone explain the above in lay terms? Is it even possible to do so!?
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  2. #2  
    Junior Member river_rat's Avatar
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    The trick with quantum computing is that the system is forced into a determinate state when you measure it, and you can be smart at change the probability of which state you will find when you measure.
    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  3. #3  
    KJW is offline
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    Quantum computing isn't about writing Word documents. It's about solving problems that require a prohibitive number of calculations. Because all the data can be placed in quantum superposition, one can perform the calculations on all the data at the same time.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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  4. #4  
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    Thanks, both. Aha, so the personal computer of the future won't be quantum-based?

    If a the data held on a qubit is revealed only at the point of measurement (and is not, as Bohr said, pre-determined), how is there any guarantee it will contain the data you saved to it!? Or am I thinking too much in terms of conventional computing? (I had read that quantum computing had applications for this, not just computing regarding high-level science and calculation).
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  5. #5  
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    Let me put it like this.

    Take a normal machine, a transistor would use a value, to compute another.

    So Value1 & Value2 -> Value3

    That is the workings of a transistor.

    A simple qubit, a good one, has multiple values. Take for instance the Symmetric, and anti-symmetric states (under parity) T and S respectivly.

    Now these have 2 values at the same time, and are in themselves a superposition of 2 possible values. But sticking to 2:

    Value1 + Value2 -> Value three.

    In short, nothing changed, but Value1 is 2 values at the same time. And so is value 2.
    In other words I've calculated 4 combinations of values at the same times, as I only calculate one with a normal transistor in fact:

    Value1 + Value2 -> Value3 \____ Value3 + Value6 = Value7
    Value4 + Value5 -> Value6 /

    Now a normal computer this would be 3 calculations. But for a quantum computer this is 4*4 = 16 calculations. 16 possible answers.
    A smart system will then be used to find the right answer. But this is very much so possible.

    It doesn't mean it is a magic system

    Some things can be done greatly with quantum computers, and some things hardly at all.
    In the age of information, ignorance is a choice.
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  6. #6  
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    Very interesting, Kerling. Thanks for your explanation.
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  7. #7  
    Junior Member tcrosa27's Avatar
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    Also, if I may add something, the key factor is the information you need to describe a system:

    if you have 2 normal bits you got: to describe this pair of bits you need 2 pieces of information, the left space (1 or 0) and the right space (1 or 0)

    Now with 2 Qubits (the outermost electron in phosphorus, spin up for 1, spin down for 0) you have the following states:
    |up up>
    |down down>
    |T0> = |1/sqrt2 (up down - down up)>
    |S> = |1/sqrt2 (up down + down up)>

    sorry for the crappy representation so I'll explain it now: there's the 2 basic states, the up up and the down down and then there's the unstable configurations, If you leave the 2 electrons with opposite states, they'll change. It's what we call an entangled state where the only property of that system is that the electrons have opposite spins but they do not have a spin of themselves these are the Singlet state (the one with the minus) and the T zero state (the one with the plus).

    This is all very fun but the point is that with 2 Qubits you need 4 pieces of information to describe a system, one for each state. And this is exponential, for 3 qubits you need 8 pieces of information. That confused me at the start but you just gotta thing about it.

    to describe 2 bits you need 2 pieces of info, ex: 01
    to describe 2 qubits you need 4 pieces of info, ex: Singlet state ---> (up down + down up)
    Science is the belief in the ignorance of experts.
    Richard Feynman
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