# Why and how is a quantum computer faster than a regular computer?

I'm currently reading a book (and alot of wikipedia) about quantum physics and I'm yet to understand how is a quantum computer can be faster than the computers we have today? what causes the possibility of a quantum computer to solve a problem of exponential time in sub-exp time?

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I found this video from Veritasium, with help from A/Prof Andrea Morello be extremely helpful in explaining this. After explaining how quantum computing works, he gives a good explanation on why quantum computing will never replace modern computing and in what cases quantum computing is slower/faster. –  Gunnar Feb 17 at 17:05
what book? plz cite it. see also how to measure processing power of a qm cpu –  vzn Feb 18 at 18:12

A quantum computer by itself isn't faster. Instead, it has a different model of computation. In this model, there are algorithms for certain (not all!) problems, which are asymptotically faster than the fastest possible (or fastest known, for some problems) classical algorithms.

I recommend reading The Limits of Quantum by Scott Aaronson: it's a short popular article explaining just what we can expect from quantum computers.

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The basic idea is that quantum devices can be in several states at the same time. Typically, a particle can have its spin up and down at the same time. This is called superposition. If you combine n particle, you can have something that can superpose $2^n$ states. Then, if you manage to extend, say, bolean operations to superposed states (or superposed symbols) you can do several computations at the same time. This has constraints but can speed up some algorithms. One major physical problem is that it is harder to maintain superposition on larger systems.

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