Near-Optimal Quantum Algorithms for String Problems

Abstract

We study quantum algorithms for several fundamental string problems, including Longest Common Substring, Lexicographically Minimal String Rotation, and Longest Square Substring. These problems have been widely studied in the stringology literature since the 1970s, and are known to be solvable by near-linear time classical algorithms. In this work, we give quantum algorithms for these problems with near-optimal query complexities and time complexities. Specifically, we show that:• Longest Common Substring can be solved by a quantum algorithm in Õ(n 2/3 ) time, improving upon the recent Õ(n 5/6 )-time algorithm by Le Gall and Seddighin (2020). Our algorithm uses the MNRS quantum walk framework, together with a careful combination of string synchronizing sets (Kempa and Kociumaka, 2019) and generalized difference covers.• Lexicographically Minimal String Rotation can be solved by a quantum algorithm in n 1/2+o(1) time, improving upon the recent Õ(n 3/4 )-time algorithm by Wang and Ying (2020). We design our algorithm by first giving a new classical divide-and-conquer algorithm in near-linear time based on exclusion rules, and then speeding it up quadratically using nested Grover search and quantum minimum finding.• Longest Square Substring can be solved by a quantum algorithm in Õ( √ n) time. Our algorithm is an adaptation of the algorithm by Le Gall and Seddighin (2020) for the Longest Palindromic Substring problem, but uses additional techniques to overcome the difficulty that binary search no longer applies.Our techniques naturally extend to other related string problems, such as Longest Repeated Substring, Longest Lyndon Substring, and Minimal Suffix.

Quantum Algorithm for the Shortest Superstring Problem

“…Some of them can be founded here [32,15,19,23,18,10]. Problems for strings are examples of such problems [17,20,30,9,1,27]. One of the most important performance metrics in this regard is query complexity; and we investigate problems using this metric for complexity.…”

Response to glassy disorder in coin on spread of quantum walker

“…A quantum version [1,2] of the classical random walk has a standard deviation of the position distribution of the walker that is linear in the number of steps [3,4]. Quantum walks (QWs) have since received a lot of attention [5][6][7][8][9][10][11][12], and have been applied to a plethora of scenarios, such as quantum algorithms [12][13][14][15][16], quantum memory [17,18], and so on [3,5,19,20]. They have also been used to model quantum state transfer [21][22][23][24].…”