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Quantum computing essentially relies on the fact that qubits maintain multiple possible states simultaneously. Parallel computing too processes multiple states simultaneously. So what is the difference or how are they different?

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  • $\begingroup$ Since no-one better informed has said anything, I shall give my impression, which is that a quantum processor maintains all possible states (for a given problem) simultaneously, while parallel computing is restricted by the number of (non-quantum) processors and/or the space available to store the states. $\endgroup$ – PJTraill May 15 '17 at 10:08
  • $\begingroup$ I don't think it's a duplicate, but I think this is definitely a related question: cs.stackexchange.com/questions/2718/… $\endgroup$ – Miles Rout May 15 '17 at 22:14
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One big difference is that in parallel computation separate processors need not be executing copies the same instruction at the same time in lockstep; they may be more loosely synchronized or in some cases completely desynchronized. In fact, separate processors need not be executing the same sequence of instructions. This is the distinction between SIMD [Single Instruction, Multiple Data] and MIMD [Multiple Instruction, Multiple Data] parallelism. Quantum computers are by nature SIMD (interpreting quantum superposition as equivalent to processor replication).

Phrased another way: With multi-core you can run all the different processes on a laptop simultaneously; you cannot do that with a quantum processor.

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    $\begingroup$ If you are not saying that quantum computing = SIMD parallel (which I would take issue with), this does nothing to answer the question. $\endgroup$ – Raphael May 10 '17 at 19:50

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