Generalization of Grover's Algorithm to Multiobject Search in Quantum   Computing, Part I: Continuous Time and Discrete Time
  L. K. Grover's search algorithm in quantum computing gives an optimal, quadratic speedup in the search for a single object in a large unsorted database. In this paper, we generalize Grover's algorithm in a Hilbert-space framework for both continuous and discrete time cases that isolates its geometrical essence to the case where more than one object satisfies the search criterion. 
Goong Chen, Stephen A. Fulling, Jeesen Chen