  In this paper, we will define a quantum operator that performs the inversion
about the mean only on a subspace of the system (Partial Diffusion Operator).
This operator is used in a quantum search algorithm that runs in O(sqrt{N/M})
for searching an unstructured list of size N with M matches such that 1<= M<=N.
We will show that the performance of the algorithm is more reliable than known
{fixed operators quantum search algorithms} especially for multiple matches
where we can get a solution after a single iteration with probability over 90%
if the number of matches is approximately more than one-third of the search
space. We will show that the algorithm will be able to handle the case where
the number of matches M is unknown in advance such that 1<=M<=N in
O(sqrt{N/M}). A performance comparison with Grover's algorithm will be
provided.
