Time- and Query-optimal Quantum Algorithms Based on Decision Trees

May 18, 2021
44 pages
Published in:
  • ACM Trans.Quant.Comput. 3 (2022) 4, 19
  • Published: Jul 27, 2022
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Abstract: (Association for Computing Machinery)
It has recently been shown that starting with a classical query algorithm (decision tree) and a guessing algorithm that tries to predict the query answers, we can design a quantum algorithm with query complexity O(√ GT where T is the query complexity of the classical algorithm (depth of the decision tree) and G is the maximum number of wrong answers by the guessing algorithm [3, 14]. In this article, we show that, given some constraints on the classical algorithms, this quantum algorithm can be implemented in time Õ(√ GT). Our algorithm is based on non-binary span programs and their efficient implementation. We conclude that various graph-theoretic problems including bipartiteness, cycle detection, and topological sort can be solved in time O(n3/2^{3/2}log2^{2}n) and with O(n3/2^{3/2}) quantum queries. Moreover, finding a maximal matching can be solved with O(n3/2^{3/2}) quantum queries in time O(n3/2^{3/2}log2^{2}n), and maximum bipartite matching can be solved in time O(n2^{2}log2^{2}n).
Note:
  • 44 pages
  • Quantum query complexity
  • quantum algorithms
  • span programs
  • quantum time complexity
  • quantum algorithm
  • topological