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quantum walk : ウィキペディア英語版 | quantum walk In quantum computing, quantum walks are the quantum analogue of classical random walks. Analogous to the classical random walk, where the walker's current state is described by a probability distribution over positions, the walker in a quantum walk is in a superposition of positions. Like classical random walks, there are two types of quantum walks: discrete-time quantum walks and continuous-time quantum walks. ==Motivation== Quantum walks are motivated by the widespread use of classical random walks in the design of randomized algorithms, and are part of several quantum algorithms. For some oracular problems, quantum walks provide an exponential speedup over any classical algorithm.〔A. M. Childs, R. Cleve, E. Deotto, E. Farhi, S. Gutmann, and D. A. Spielman, Exponential algorithmic speedup by quantum walk, Proc. 35th ACM Symposium on Theory of Computing, pp. 59–68, 2003, quant-ph/0209131.〕〔A. M. Childs, L. J. Schulman, and U. V. Vazirani, Quantum algorithms for hidden nonlinear structures, Proc. 48th IEEE Symposium on Foundations of Computer Science, pp. 395–404, 2007, arXiv:0705.2784.〕 Quantum walks also give polynomial speedups over classical algorithms for many practical problems, such as the element distinctness problem,〔Andris Ambainis, Quantum walk algorithm for element distinctness, SIAM J. Comput. 37 (2007), no. 1, 210–239, , preliminary version in FOCS 2004.〕 the triangle finding problem,〔F. Magniez, M. Santha, and M. Szegedy, Quantum algorithms for the triangle problem, Proc. 16th ACM-SIAM Symposium on Discrete Algorithms, pp. 1109–1117, 2005, quant-ph/0310134.〕 and evaluating NAND trees.〔E. Farhi, J. Goldstone, and S. Gutmann, A quantum algorithm for the Hamiltonian NAND tree, Theory of Computing 4 (2008), no. 1, 169–190, quant-ph/0702144〕 The well-known Grover search algorithm can also be viewed as a quantum walk algorithm.
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