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We prove a characterization of quantum query algorithms in terms of polynomials satisfying a certain (completely bounded) norm constraint. Based on this, we obtain a refined notion of approximate polynomial degree that equals the quantum query complexity, answering a question of Aaronson et al. (CCC'16). Using this characterization, we show that many polynomials of degree at least 4 are far from those coming from quantum query algorithms. Our proof is based on a fundamental result of Christensen and Sinclair (J. Funct. Anal., 1987) that generalizes the well-known Stinespring representation for quantum channels to multilinear forms. We also give a simple and short proof of one of the results of Aaronson et al. showing an equivalence between one-query quantum algorithms and bounded quadratic polynomials.
@InProceedings{arunachalam_et_al:LIPIcs.ITCS.2018.3,
author = {Arunachalam, Srinivasan and Bri\"{e}t, Jop and Palazuelos, Carlos},
title = {{Quantum Query Algorithms are Completely Bounded Forms}},
booktitle = {9th Innovations in Theoretical Computer Science Conference (ITCS 2018)},
pages = {3:1--3:21},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-060-6},
ISSN = {1868-8969},
year = {2018},
volume = {94},
editor = {Karlin, Anna R.},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2018.3},
URN = {urn:nbn:de:0030-drops-83383},
doi = {10.4230/LIPIcs.ITCS.2018.3},
annote = {Keywords: Quantum query algorithms, operator space theory, polynomial method, approximate degree.}
}