Directed st-connectivity with few paths is in quantum logspace
<|reference_start|>Directed st-connectivity with few paths is in quantum logspace: We present a $\mathsf{BQSPACE}(O(\log n))$-procedure to count $st$-paths on directed graphs for which we are promised that there are at most polynomially many paths starting in $s$ and polynomially many paths ending in $t$. For comparison, the best known classical upper bound in this case just to decide $st$-connectivity is $\mathsf{DSPACE}(O(\log^2 n/ \log \log n))$. The result establishes a new relationship between $\mathsf{BQL}$ and unambiguity and fewness subclasses of $\mathsf{NL}$. Further, some preprocessing in our approach also allows us to verify whether there are at most polynomially many paths between any two nodes in $\mathsf{BQSPACE}(O(\log n))$. This yields the first natural candidate for a language problem separating $\mathsf{BQL}$ from $\mathsf{L}$ and $\mathsf{BPL}$. Until now, all candidates separating these classes were promise problems.<|reference_end|>
@article{edenhofer2024directed,
    title={Directed st-connectivity with few paths is in quantum logspace},
    author={Roman Edenhofer, Simon Apers},
    journal={arXiv preprint arXiv:2408.12473},
    year={2024},
    archivePrefix={arXiv},
    eprint={2408.12473},
    primaryClass={quant-ph cs.CC cs.DS}
}