Every NAND formula of size N can be evaluated in time N1/2+o(1) on a quantum computer
Abstract
For every NAND formula of size N, there is a bounded-error N1/2+o(1)-time quantum algorithm, based on a coined quantum walk, that evaluates this formula on a black-box input. Balanced, or ``approximately balanced,'' NAND formulas can be evaluated in O(sqrtN) queries, which is optimal. It follows that the (2-o(1))-th power of the quantum query complexity is a lower bound on the formula size, almost solving in the positive an open problem posed by Laplante, Lee and Szegedy.
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