Any AND-OR Formula of Size $N$ Can Be Evaluated in Time $N^{1/2+o(1)}$ on a Quantum Computer

  • Authors:
  • A. Ambainis;A. M. Childs;B. W. Reichardt;R. Špalek;S. Zhang

  • Affiliations:
  • ambainis@lu.lv;amchilds@uwaterloo.ca;breic@uwaterloo.ca;spalek@google.com;syzhang@cse.cuhk.edu.hk

  • Venue:
  • SIAM Journal on Computing
  • Year:
  • 2010

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Abstract

Consider the problem of evaluating an AND-OR formula on an $N$-bit black-box input. We present a bounded-error quantum algorithm that solves this problem in time $N^{1/2+o(1)}$. In particular, approximately balanced formulas can be evaluated in $O(\sqrt{N})$ queries, which is optimal. The idea of the algorithm is to apply phase estimation to a discrete-time quantum walk on a weighted tree whose spectrum encodes the value of the formula.