Quantum Approximate Counting for Markov Chains and Application to Collision Counting

In this paper we show how to generalize the quantum approximate counting technique developed by Brassard, Høyer and Tapp [ICALP 1998] to a more general setting: estimating the number of marked states of a Markov chain (a Markov chain can be seen as a random walk over a graph with weighted edges). This makes it possible to construct quantum approximate counting algorithms from quantum search algorithms based on the powerful "quantum walk based search" framework established by Magniez, Nayak, Roland and Santha [SIAM Journal on Computing 2011]. As an application, we apply this approach to the quantum element distinctness algorithm by Ambainis [SIAM Journal on Computing 2007]: for two injective functions over a set of $N$ elements, we obtain a quantum algorithm that estimates the number $m$ of collisions of the two functions within relative error $ε$ by making $\tilde{O}\left(\frac{1}{ε^{25/24}}\big(\frac{N}{\sqrt{m}}\big)^{2/3}\right)$ queries, which gives an improvement over the $Θ\big(\frac{1}ε\frac{N}{\sqrt{m}}\big)$-query classical algorithm based on random sampling when $m\ll N$.