Physics of Computation and the Quantum Computing Challenge the Spaghetti Problem the Cooking Problem Quantum Coherent Parallel Computation

Introduction New computation devices increasingly depend on particular physical properties rather than on logical organization alone as used to be the case in conventional technologies. The laws of physics impose limits on increases in computing power. Two of these limits are interconnect wires in multicomputers and thermodynamic limits to energy dissipation in all computers. Quantum computing is a new computational technology which promises to eliminate problems of latency and wiring associated with parallel computers and the rapidly approaching ultimate limits to computing power imposed by the fundamental thermodynamics. It opens the prospect for unlimited parallelism and has been shown to work fast for some standard problems like factoring, where all known classical methods use exponentially exploding time. Quantum search algorithms for unstructured databases turn out to be quadratic faster than classical algorithms can ever be. There are tremendous economical and societal interests involved in the ultimate realization of quantum computing. Here we sketch the short history, basic principles, and implications of this emerging area. In The Netherlands CWI has been one of the first to be involved. In nonsequential (parallel or distributed) computation one can hardly ignore the physical aspects of the underlying computer system. Complexity analysis may stand or fall with the account taken of physical reality. Moreover, nonclassical or nonstandard physical realizations of computers may have totally unexpected properties. Nonconventional technologies which may yield novel computation opportunities, include quantum cryptography [1] and quantum coherent computation. Because the latter, if physically realizable, allows breaking most commonly used cryptosystems, quantum cryptography may be the only safe principle for public cryptography, in the sense that its safety rests on the validity of quantum mechanics, rather than on unproven cryptographic assumptions.