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Can quantum computer provide random or just pseudo-random number, or none of both? It's a bit confusing me, since collapse of wave function once measured.

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  • $\begingroup$ Isn't this just up to whether a QC must be programmed with an algorithm to create this number, or not? One definition of a random number is a number that has no apparent algorithm associated with its creation. If a QC can produce numbers without our input, I have a feeling they are random in this definition. $\endgroup$ – obliv Jul 7 '16 at 14:24
  • $\begingroup$ You have to define exactly what you mean by 'random'. $\endgroup$ – lemon Jul 7 '16 at 14:40
  • $\begingroup$ @lemon: random = independent to status of any other universe entity. $\endgroup$ – Leon Kigelman Jul 7 '16 at 14:42
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    $\begingroup$ @LeonKigelman then the answer is we don't know. See: Can randomness exist? and links therein. $\endgroup$ – lemon Jul 7 '16 at 14:54
  • $\begingroup$ A QC can only produce numbers that are correlated with some other system (usually called "measurement device"). If you can be sure that whatever you are doing is in some form "independent" of said measurement device, then the numbers may be "random enough" for your purpose. If that is not the case, then they can be perfectly correlated. A famous example is entanglement, where each individual measurement looks like a random sequence, but the correlation between the two measurements can be 100% correlated. $\endgroup$ – CuriousOne Jul 7 '16 at 23:01
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Both. If quantum mechanics is correct, then an unpredictable RNG circuit is extremely simple:

rng circuit

(Note: even if you're a proponent of a deterministic interpretation, like many worlds, this circuit remains fundamentally unpredictable. Philosophers may debate whether or not it's "really" random, but predictability is ultimately what matters in practice for cryptography and so forth.)

It gets better, actually. Thanks to the existence of Bell tests, we can use quantum computers to create so-called "Einstein-certified" random numbers that are guaranteed to be unpredictable unless faster-than-light communication was used to bias them.

For more information see the well-written article 'Quantum Randomness' by Scott Aaronson.

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  • $\begingroup$ This is not necessarily random in the sense that Leon defines above. $\endgroup$ – lemon Jul 7 '16 at 14:53
  • $\begingroup$ @lemon They say they want " independent to status of any other universe entity". The presented circuit satisfies that property: it won't correlate with any other observable. If it does correlate with other observables, then quantum mechanics is wrong (though perhaps only in some minor way). $\endgroup$ – Craig Gidney Jul 7 '16 at 14:56
  • $\begingroup$ I take 'universe element' to not necessarily mean an observable. For instance, if we embrace non-locality then we can keep both QM and local hidden variables. $\endgroup$ – lemon Jul 7 '16 at 15:08
  • $\begingroup$ @lemon I'm not going to worry about unobservable things in my answer. $\endgroup$ – Craig Gidney Jul 7 '16 at 15:10

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