What is the difference between normal computers and Qunatum Computers, why are they so much faster/more efficient?
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There are some types of problems where it’s hard to *find* the answer but easy to *check* a possible guess. The classic example is factoring large numbers. If I ask you “find a four-digit number that divides 47,086,973”, the only way to do that is trial and error, guessing and checking four-digit numbers one at a time until you eventually find one that works. However, each individual check–e.g. “check whether 47,086,973 is divisible by 6203”–is easy; the hard part is just that there are a lot of possible four-digit numbers to try.
A quantum computer is designed to work simultaneously in multiple parallel universes. We tell the computer “*randomly* pick a four-digit number and then check whether it divides 47,086,973”. It picks a different random number in different timelines. In most of those timelines the answer is “no”, but in a few the answer is “yes”. If we can arrange to inspect a version of the computer that got a “yes” answer, then from our perspective it will have had a lucky guess on its first try. That’s what makes it faster than a classical computer: a classical computer won’t normally guess right on its first try, but will instead have to take many guesses before it finds one that works.
The tricky part of this–well, one of several tricky parts–is that there can only ever be one version of the computer in our timeline. So it’s very important not to interact with the computer in any way that would be possible if it *hadn’t* found the correct answer. Do not look at the computer, do not touch the computer, do not receive heat from the computer, etc.; do not do anything except receive an “I guessed right!” message from the computer. Non-interaction is hard; currently we can only do it with very small objects for very brief periods of time in very cold rooms, which makes quantum computing impractical; quantum computers are still something we’re *studying* rather than something we’re *using*.
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