A still largely theoretical but eagerly researched breed of Computers which use quantum superposition to deal simultaneously with all possible values which can be represented as "qubits" (quantum bits, as opposed to conventional computer bits representing 0 or 1) in a quantum register. Experimental devices with small numbers of qubits have been tested; if workable on a larger scale, the technology should be capable of processing speeds vastly greater than normal computers, potentially solving now-uncomputable problems such as the strongest current encryption. In sf, quantum computers provide a convenient shorthand for the kind of extraordinary computing power that may be required to sustain true AI, as in Greg Bear's Moving Mars (1994) with its QL (quantum logic) "thinkers", or in Peter Watts's Blindsight (2006).
Because quantum computers can be thought of as operating simultaneously in all the branching realities implied by the many-worlds interpretation of quantum theory, a tempting – though highly dubious – sf speculation is that they may manipulate or allow transfer between Parallel Worlds: this conceit is explored in such novels as Ian Watson's Hard Questions (1996) and Ian McDonald's Brasyl (2007). [DRL]
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