Span programs and quantum algorithms for st-connectivity and claw detection

TL;DR

This paper develops quantum algorithms for graph connectivity and subgraph detection using span programs, achieving query complexities of O(n d^{1/2}) and O(n), with efficient implementation and novel span program modifications.

Contribution

It introduces span programs for st-connectivity and subgraph detection, leading to new quantum algorithms with improved query complexities for these problems.

Findings

Quantum algorithms for st-connectivity with O(n d^{1/2}) queries.

Detection of specific subgraphs with O(n) queries.

Efficient implementation in time and space for most algorithms.

Abstract

We introduce a span program that decides st-connectivity, and generalize the span program to develop quantum algorithms for several graph problems. First, we give an algorithm for st-connectivity that uses O(n d^{1/2}) quantum queries to the n x n adjacency matrix to decide if vertices s and t are connected, under the promise that they either are connected by a path of length at most d, or are disconnected. We also show that if T is a path, a star with two subdivided legs, or a subdivision of a claw, its presence as a subgraph in the input graph G can be detected with O(n) quantum queries to the adjacency matrix. Under the promise that G either contains T as a subgraph or does not contain T as a minor, we give O(n)-query quantum algorithms for detecting T either a triangle or a subdivision of a star. All these algorithms can be implemented time efficiently and, except for the triangle-detection algorithm, in logarithmic space. One of the main techniques is to modify the st-connectivity span program to drop along the way "breadcrumbs," which must be retrieved before the path from s is allowed to enter t.