Expansion Testing using Quantum Fast-Forwarding and Seed Sets

Expansion testing aims to decide whether an $n$-node graph has expansion at least $Phi$, or is far from any such graph. We propose a quantum expansion tester with complexity $widetilde{O}(n^{1/3}Phi^{-1})$. This accelerates the $widetilde{O}(n^{1/2}Phi^{-2})$ classical tester by Goldreich and Ron [Algorithmica 02], and combines the $widetilde{O}(n^{1/3}Phi^{-2})$ and $widetilde{O}(n^{1/2}Phi^{-1})$ quantum speedups by Ambainis, Childs and Liu [RANDOM 11] and Apers and Sarlette [QIC 19], respectively. The latter approach builds on a quantum fast-forwarding scheme, which we improve upon by initially growing a seed set in the graph. To grow this seed set we use a so-called evolving set process from the graph clustering literature, which allows to grow an appropriately local seed set.