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Sculpting Quantum Speedups

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Abstract

Given a problem which is intractable for both quantum and classical algorithms, can we find a sub-problem for which quantum algorithms provide an exponential advantage? We refer to this problem as the "sculpting problem." In this work, we give a full characterization of sculptable functions in the query complexity setting. We show that a total function f can be restricted to a promise P such that Q(f|_P)=O(polylog(N)) and R(f|_P)=N^{Omega(1)}, if and only if f has a large number of inputs with large certificate complexity. The proof uses some interesting techniques: for one direction, we introduce new relationships between randomized and quantum query complexity in various settings, and for the other direction, we use a recent result from communication complexity due to Klartag and Regev. We also characterize sculpting for other query complexity measures, such as R(f) vs. R_0(f) and R_0(f) vs. D(f). Along the way, we prove some new relationships for quantum query complexity: for example, a nearly quadratic relationship between Q(f) and D(f) whenever the promise of f is small. This contrasts with the recent super-quadratic query complexity separations, showing that the maximum gap between classical and quantum query complexities is indeed quadratic in various settings - just not for total functions! Lastly, we investigate sculpting in the Turing machine model. We show that if there is any BPP-bi-immune language in BQP, then every language outside BPP can be restricted to a promise which places it in PromiseBQP but not in PromiseBPP. Under a weaker assumption, that some problem in BQP is hard on average for P/poly, we show that every paddable language outside BPP is sculptable in this way.

BibTeX - Entry

@InProceedings{aaronson_et_al:LIPIcs:2016:5853,
  author =	{Scott Aaronson and Shalev Ben-David},
  title =	{{Sculpting Quantum Speedups}},
  booktitle =	{31st Conference on Computational Complexity (CCC 2016)},
  pages =	{26:1--26:28},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-008-8},
  ISSN =	{1868-8969},
  year =	{2016},
  volume =	{50},
  editor =	{Ran Raz},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum fuer Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{http://drops.dagstuhl.de/opus/volltexte/2016/5853},
  URN =		{urn:nbn:de:0030-drops-58538},
  doi =		{10.4230/LIPIcs.CCC.2016.26},
  annote =	{Keywords: Quantum Computing, Query Complexity, Decision Tree Complexity, Structural Complexity}
}

Keywords: Quantum Computing, Query Complexity, Decision Tree Complexity, Structural Complexity
Seminar: 31st Conference on Computational Complexity (CCC 2016)
Issue date: 2016
Date of publication: 2016


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