Quantum computing isacomputer scientist's dream. By exploiting the ability of a quantum system such as an array of atoms to be in many different energy states at once, a quantum computer can, in theory, perform vast numbers of computations at the sametime, tackling problems that would overwhelm conventional machines. But so far, this fantasy has remained just that: a fantasy. No one has built a quantum computer, much less programmed one to calculate anything. And one bite of reality keeping computerscientists from realizing this fantasy has been the notorious fragility of quantum states, which makes quantum systems vulnerable to errors.
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