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I Why must qbits be entangled?

  1. Apr 25, 2017 #1
    Hello

    I understand what entanglement is and also what superposition is.
    But I don't understand why qbits in a quantum computer must be entangled.
     
  2. jcsd
  3. Apr 25, 2017 #2

    Strilanc

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    They don't have to be entangled. But if you never entangle them, you might as well use a classical computer instead. It's easy to classically simulate quantum circuits that don't entangle qubits.
     
  4. Apr 25, 2017 #3
    OK, What are the benefits of entangle the qbits?
     
  5. Apr 25, 2017 #4

    phinds

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    Have you done ANY research into quantum computers?
     
  6. Apr 26, 2017 #5
    No, I've just read a little bit.
     
  7. Apr 26, 2017 #6

    phinds

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    Then I suggest you read a bit more and ask questions when you hit something you don't understand.
     
  8. Apr 27, 2017 #7
    That's what I don't understand. Why they have to be entangled.

    I passed my exams on quantum mechanics some years ago, and I've read some sparse articles about quantum computers, annealing... and even wathched many videos on youtube about QC.
    I understand all the basic concepts but I don't see why is entanglement a necessary condition.
     
  9. Apr 27, 2017 #8

    f95toli

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    You don't HAVE to entangle the qubits. There are plenty of single qubit gate operations, but you can't really do anything very useful with those (at least not if you want to compute something).
    If you look at some of the more useful algorithms (say Grover's algorithm) you will find that they DO require that the qubits are entangled in order to work.

    Or in other words: entanglement is the extra "resource" that these algorithms use in order to achieve a speedup.
     
  10. Apr 27, 2017 #9
    Hello.

    @f95toli
    Where can I read more details about why this happens?
     
  11. Apr 28, 2017 #10

    f95toli

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    I don't think I understand the question.
    If you are asking why entanglement leads to speedup I don't think there is general answer to that question since it only leads to a speedup in some very specific cases, quantum computers are only faster than classical computers when you can use an algorithm that somehow can use entangled states to do something efficiently (say compute a Fourier transform).
    Have you tried looking at e.g. the Wiki for Grover's algorithm? If you want to understand this in detail you need to get e.g. Nielsen&Chuang's book.
     
  12. Apr 30, 2017 #11
    Entangling qbits allows you to "tag" a particular value in a calculation, which you cannot readout but can do operations on. For example in Grover's algorithm you can identify the correct item in a list and invert it without reading its value. The reading of its value is done at the end when the computation is finished.
     
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