A quantum no-reflection theorem and the speeding up of Grover's search   algorithm
  We prove that it is impossible to built a universal quantum machine that produces reflections about an unknown state. We then point out a connection between this result and the optimality of Grover's search algorithm: if such reflection machines were available, it would be possible to accelerate Grover's search algorithm to exponential speedups. 
Karthikeyan S. Kumar and G. S. Paraoanu
quant-ph