Summary
Graphs are collections of dots connected by lines, and sometimes we want to test if their dots can be separated into two groups with no connections inside each group (bipartiteness) or if they are well connected overall (expansion). Classical computers need about the square root of the number of dots to test these properties, but quantum computers can do better. The authors found nearly the best possible lower limits on how many queries quantum computers must make for these tests, showing no major improvement is possible beyond what is already known. They did this by translating the problem into simpler ones and analyzing polynomial techniques more precisely.
What this means in practice
- •For quantum algorithm designers: Know the fundamental limits of quantum speedup for testing certain graph properties when designing new quantum algorithms.
- •For network analysts: Understand that quantum speedups for testing network bipartiteness or expansion are limited, affecting expectations for quantum tools in network diagnostics.
A theory result. No direct application yet.
Abstract
In this work, we study bipartiteness and expansion testing, two canonical problems in graph property testing in the bounded-degree model through the lens of quantum query complexity. In the classical setting, it is known that $\widetildeΘ(\sqrt{N})$ queries are necessary and sufficient for both these testing problems (Goldreich and Ron, 1999, 2000 & 2002), where $N$ denotes the number of vertices of the input graph. Due to their significance, (Ambainis, Childs, and Liu, 2011) initiated the study of these problems in the quantum setting and designed quantum algorithms for bipartiteness and expansion testing that perform $\widetilde{O}(N^{1/3})$ queries, showing a polynomial speedup. They also proved that $\widetildeΩ(N^{1/4})$ queries are necessary for expansion testing, but the possibility of an exponential quantum advantage for bipartiteness testing remained open. Despite significant effort, there has been no improvement in these results in the last decade and a half. In this work, we prove essentially tight $\widetildeΩ(N^{1/3})$ quantum query lower bounds for both bipartiteness and expansion testing, thereby completely characterizing the quantum query complexity of these problems up to polylogarithmic factors. While our proofs use the polynomial method similarly to Ambainis, Childs, and Liu, we use intermediate problems that we relate to the main problems via reductions, and perform a more precise analysis of the resulting polynomials, leading to the near-optimal lower bounds.