A Limit on the Speed of Quantum Computation in Determining Parity
  Consider a function f which is defined on the integers from 1 to N and takes the values -1 and +1. The parity of f is the product over all x from 1 to N of f(x). With no further information about f, to classically determine the parity of f requires N calls of the function f. We show that any quantum algorithm capable of determining the parity of f contains at least N/2 applications of the unitary operator which evaluates f. Thus for this problem, quantum computers cannot outperform classical computers. 
E. Farhi (MIT), J. Goldstone (MIT), S. Gutmann (Northeastern), M.   Sipser (MIT)