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The polynomial and the adversary methods are the two main tools for proving lower bounds on query complexity of quantum algorithms. Both methods have found a large number of applications, some problems more suitable for one method, some for the other. It is known though that the adversary method, in its general negative-weighted version, is tight for bounded-error quantum algorithms, whereas the polynomial method is not. By the tightness of the former, for any polynomial lower bound, there ought to exist a corresponding adversary lower bound. However, direct reduction was not known. In this paper, we give a simple and direct reduction from the polynomial method (in the form of a dual polynomial) to the adversary method. This shows that any lower bound in the form of a dual polynomial is actually an adversary lower bound of a specific form.
@InProceedings{belovs:LIPIcs.TQC.2024.11,
author = {Belovs, Aleksandrs},
title = {{A Direct Reduction from the Polynomial to the Adversary Method}},
booktitle = {19th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2024)},
pages = {11:1--11:15},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-328-7},
ISSN = {1868-8969},
year = {2024},
volume = {310},
editor = {Magniez, Fr\'{e}d\'{e}ric and Grilo, Alex Bredariol},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.TQC.2024.11},
URN = {urn:nbn:de:0030-drops-206814},
doi = {10.4230/LIPIcs.TQC.2024.11},
annote = {Keywords: Polynomials, Quantum Adversary Bound}
}