Using a quantum computer, one could dramatically accelerate the simulation of quantum physics and chemistry (the original application advocated by Richard Feynman in the 1980s), break almost all of the public-key cryptography currently used on the Internet (for example, by quickly factoring large numbers with the famous Shor's algorithm 1) and maybe achieve a modest speed-up for solving optimization problems in the infamous NP-hard class (but no one is sure about the last one). Supposing we had a quantum computer, what would we use it for? The 'killer apps' - the applications for which a quantum computer would promise huge speed advantages over classical computers - have struck some people as inconveniently narrow. But, there's always been a catch - and I'm not even talking about the difficulty of building practical quantum computers.

Not only would a quantum computer harness the notorious weirdness of quantum mechanics, but it would do so for a practical purpose: solving certain problems exponentially faster than we know how to solve them with any existing computer. For twenty years, quantum computing has been catnip to science journalists.

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