Longest common substring (LCS) is an important text processing problem, which has recently been investigated in the quantum query model. The decision version of this problem, LCS with threshold d, asks whether two length-n input strings have a common substring of length d. The two extreme cases, d=1 and d=n, correspond, respectively to Element Distinctness and Unstructured Search, two fundamental problems in quantum query complexity. However, the intermediate case 1 << d << n was not fully understood. We show that the complexity of LCS with threshold d smoothly interpolates between the two extreme cases up to no(1) factors: -LCS with threshold d has a quantum algorithm in n(2/3+o(1))/d(1/6) query complexity and time complexity, and requires at least Omega(n(2/3)/d(1/6)) quantum query complexity. Our result improves upon previous upper bounds O(min{n/d(1/2),n(2/3)}) (Le Gall and Seddighin ITCS 2022, Akmal and Jin SODA 2022), and answers an open question of Akmal and Jin. Our main technical contribution is a quantum speed-up of the powerful String Synchronizing Set technique introduced by Kempa and Kociumaka (STOC 2019). It consistently samples n/tau 1-o(1) synchronizing positions in the string depending on their length-Theta(tau) contexts, and each synchronizing position can be reported by a quantum algorithm in O(tau(1/2+o(1))) time. Our quantum string synchronizing set also yields a near-optimal LCE data structure in the quantum setting. As another application of our quantum string synchronizing set, we study the k -mismatch Matching problem, which asks if the pattern has an occurrence in the text with at most k Hamming mismatches. Using a structural result of Charalampopoulos et al. (FOCS 2020), we obtain: -k-mismatch matching has a quantum algorithm with k(3/4)n(1/2+o(1)) query complexity and O(kn(1/2)) time complexity. We also observe a non-matching quantum query lower bound of Omega(root kn).
