A quantum algorithm for finding the modal value*

We present a quantum algorithm for finding the most often occurring (or modal) value of a data set. We thereby supplement other algorithms that can determine the mean value or similar quantities. Our algorithm [1] requires the combined use of quantum counting and extended quantum search, and gives a quadratic speed up over the classical situation. For a data list of N elements, each entry an integer in the range [1,d], our method requires O(d N$^{1/2})$ oracle calls, and further complexity results are described. [1] to appear in Quantum Information Processing.