For a "new technology" computer to be useful, one needs to be able to
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conveniently prepare inputs,
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conveniently program,
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reliably produce answer (quicker than other techniques), and
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conveniently read out answer
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Conventional computers are built around bit ( taking values 0 or 1) manipulation
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One can build arbitarily complex arithmetic if have some way of implementing NOT and AND
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Quantum Systems naturally represent bits
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A spin (of say an electron or proton) is either up or down
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A hydrogen atom is either in lowest or (first) excited state etc.
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