Phase matching in quantum searching

Each iteration in Grover's original quantum search algorithm contains 4 steps: two Hadamard-Walsh transformations and two amplitudes inversions. When the inversion of the marked state is replaced by arbitrary phase rotation \theta and the inversion for the prepared state |\gamma> is replaced by rotation through \phi, we found that these phase rotations must satisfy a matching condition \theta=\phi. Approximate formula for the amplitude of the marked state after an arbitrary number of iterations are also derived. We give also a simple explanation of the phase matching requirement.