Abstract

Let X = (z,, , z,-,) be a sequence of n numbers. For 6> 0, we say that 5; is an e-approximate median if the num-ber of elements strictly less than zi and the number of elements strictly greater than zi are each less than (1 + 6):. We consider the quantum query complexity of computing an c-approximate median, given the sequence X as an oracle. We prove a lower bound of n(min{t,n}) queries for any quantum algorithm that computes an r-approximate median with any constant probability greater than l/2. We also show how an c-approximate median may be computed with 0 ( $ log(t) log log ( $)) oracle queries, which rep resents an improvement over an earlier algorithm due to Grover [ll, 121. Thus, the lower bound we obtain is essentially optimal. The upper and the lower bound both hold in the comparison tree model as well. Our lower bound result is an application of the polynomial paradigm recently introduced to quantum complexity theory by Be & et ol. [l]. The main ingredient in the proof is a polynomial degree lower bound far real multilinear polynomials that “approximate” symmetric partial boolean functions. The degree bound extends a result of Patti [15] and also immediately yields lower bounds for the problems of approximating the kth-smallest element, approximating the mean of a sequence of numbers, and approximately counting the number of ones of a boolean function. All bounds obtained come within a polylogarithmic factor of the optimal (as we show by presenting algorithms where no such optimal or near optimal algorithms were known), thus demonstrating the power of the polynomial method.

Citations