Recently a great deal of attention has been focused on quantum computation following a sequence of results [Bernstein and Vazirani, in Proc. 25th Annual ACM Symposium Theory Comput., 1993, pp. 11–20, SIAM J. Comput., 26 (1997), pp. 1411–1473], [Simon, in Proc. 35th Annual IEEE Symposium Foundations Comput. Sci., 1994, pp. 116–123, SIAM J. Comput., 26 (1997), pp. 1474–1483], [Shor, in Proc. 35th...