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This work describes a new algorithm for creating a superposition over the edge set of a graph, encoding a quantum sample of the random walk stationary distribution. The algorithm requires a number of quantum walk steps scaling as O~(m^(1/3) delta^(-1/3)), with m the number of edges and delta the random walk spectral gap. This improves on existing strategies by initially growing a classical seed set in the graph, from which a quantum walk is then run. The algorithm leads to a number of improvements: (i) it provides a new bound on the setup cost of quantum walk search algorithms, (ii) it yields a new algorithm for st-connectivity, and (iii) it allows to create a superposition over the isomorphisms of an n-node graph in time O~(2^(n/3)), surpassing the Omega(2^(n/2)) barrier set by index erasure.
@InProceedings{apers:LIPIcs.ESA.2019.9,
author = {Apers, Simon},
title = {{Quantum Walk Sampling by Growing Seed Sets}},
booktitle = {27th Annual European Symposium on Algorithms (ESA 2019)},
pages = {9:1--9:12},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-124-5},
ISSN = {1868-8969},
year = {2019},
volume = {144},
editor = {Bender, Michael A. and Svensson, Ola and Herman, Grzegorz},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2019.9},
URN = {urn:nbn:de:0030-drops-111300},
doi = {10.4230/LIPIcs.ESA.2019.9},
annote = {Keywords: Quantum algorithms, Quantum walks, Connectivity, Graph theory}
}