Multidimensional Quantum Walks, with Application to $k$-Distinctness

While the quantum query complexity of $k$-distinctness is known to be $O\left(n^{{3/4-1/4(2k-1)}\right)$} for any constant $k \geq 4$, the best previous upper bound on the time complexity was $\widetilde{O}\left(n^{{1-1/k}\right)$.} We give a new upper bound of $\widetilde{O}\left(n^{{3/4-1/4(2k-1)}\right)$} on the time complexity, matching the query complexity up to polylogarithmic factors. In order to achieve this upper bound, we give a new technique for designing quantum walk search algorithms, which is an extension of the electric network framework. We also show how to solve the welded trees problem in $O(n)$ queries and $O(n^2)$ time using this new technique, showing that the new quantum walk framework can achieve exponential speedups.