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Suppose you have a large collection of N numbered copper coins. However, one of them is nickel but looks exactly the same as the rest. How do you find out which number the nickel coin has?

Classical computer: Try each of them in turn to see if it is magnetic, this takes on average N/2 tries.

Quantum computer: Put them all in a bag together with a magnet on a string, shake a bit and pull out the magnet plus the nickel coin. If it doesn't work the first time try again. This takes only one or a few tries. The magnet interacts with all the coins at once.

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    $\begingroup$ So...what's the question? The analogy is interesting...but like all analogies...it's flawed. $\endgroup$ – Sanchayan Dutta Jun 21 '19 at 13:32
  • $\begingroup$ Of course its not perfect. I was thinking of using it to explain QC to an audience that understands it even less than I do.... It highlights the parallel character of QC. A question might be whether you have an even better metaphor. $\endgroup$ – michiel perdeck Jun 21 '19 at 13:34
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    $\begingroup$ This vaguely describes something like Grover's algorithm. It doesn't really say anything about quantum computing in general so far as I can see. The magnet 'interacts' with all coins at once, but what does the attractive magnetic force correspond to? What features of Shor's algorithm does this analogy describe? It's marginally better than "trying all possibilities at once", but that's because "trying all possibilities at once" is a terrible explanation. The question is whether you want to explain quantum computing, however broadly, or whether you're happy with just seeming to explain. $\endgroup$ – Niel de Beaudrap Jun 21 '19 at 16:06
  • $\begingroup$ It is I think somewhat similar to a chemical reaction, also a quantum process although indeed not a QC algorithm. But I would love to hear about better metaphors to use. $\endgroup$ – michiel perdeck Jun 21 '19 at 16:10
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    $\begingroup$ It is possible that no good metaphors exist. (We might find them, and people are still trying, but it's not an easy thing.) To consider a similar question: what metaphor would you use to describe how the magnet itself works, that captures the fact that there are two poles? For magnets, we get around the problem by assuming that people have a practical knowledge of what magnets are like: like dogs and apples, we don't require a lay-persons explanation of them because people know what they are. But quantum phenomena (such as quantum computing) are neither obvious nor accessible in this way... $\endgroup$ – Niel de Beaudrap Jun 21 '19 at 18:13
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Why not uncover the quantum computer—open the box—to reveal the mechanism? Well, we can’t. If we “watch” the computation happen, we expose the quantum computer to an environment and this will break the computation. The kind of things a quantum computer needs to do requires complete isolation from the environment. Just like a magician’s trick, if we reveal the mechanism, the magic doesn’t happen.

Don’t place your analogy at the level of explanation — place it at the level of the phenomenon. Let your analogy do the work of explanation for you.

Hope you find this useful

If you really want to explain quantum computing and its workings you will have to explain a little math, which hopefully isn’t too much for your audience.

Just keep in mind that the math won't fully help you understand how the mechanism works, it is simply just a better way of putting the concept of quantum computing and for a better understanding of the concept behind it. A good metaphor can be found in this answer.

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