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?

  • $\begingroup$ 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, 2017 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, 2017 at 22:14
  • $\begingroup$ cs.stackexchange.com/q/12892/755, cs.stackexchange.com/q/48045/755 $\endgroup$
    – D.W.
    Sep 3, 2020 at 17:13

2 Answers 2


For a parallel computer, we need to have one billion different processors. In a quantum computer, a single register can perform a billion computations. This is known as quantum parallelism. Also a quantum computer returns a single randomly-selected correct result, while a parallel computer can directly return all valid results. That's the reason why reading results out of quantum computers generally is slow and getting all results requires running the quantum computation enough times to be relatively certain that the random sampling has seen all possible results.

Hope this helps!

See this also: differences-between-quantum-computing-and-parallelism

  • 1
    $\begingroup$ "In a quantum computer, all one billion computations will be running on the same hardware" is not exactly how I would put it. Similarly, "a quantum computer simply returns a single randomly-selected correct result" seems like a bad oversimplification. But I do appreciate that these simplifications may help for very unfamiliar people to start to grasp the ideas, I just think they may lead to misconceptions. $\endgroup$
    – 6005
    Sep 4, 2020 at 12:59
  • $\begingroup$ I think that's why i've provided the link. Still i'll edit my answer. $\endgroup$ Sep 4, 2020 at 17:24

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.

  • 1
    $\begingroup$ 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, 2017 at 19:50

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