The Acrobatics of BQP

Authors Scott Aaronson, DeVon Ingram, William Kretschmer



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Scott Aaronson
  • University of Texas at Austin, TX, USA
DeVon Ingram
  • University of Chicago, IL, USA
William Kretschmer
  • University of Texas at Austin, TX, USA

Acknowledgements

We thank Lance Fortnow, Greg Kuperberg, Patrick Rall, and Avishay Tal for helpful conversations. We are especially grateful to Avishay Tal for helping us prove a tail bound on the block sensitivity of AC⁰ circuits.

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Scott Aaronson, DeVon Ingram, and William Kretschmer. The Acrobatics of BQP. In 37th Computational Complexity Conference (CCC 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 234, pp. 20:1-20:17, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022) https://doi.org/10.4230/LIPIcs.CCC.2022.20

Abstract

One can fix the randomness used by a randomized algorithm, but there is no analogous notion of fixing the quantumness used by a quantum algorithm. Underscoring this fundamental difference, we show that, in the black-box setting, the behavior of quantum polynomial-time (BQP) can be remarkably decoupled from that of classical complexity classes like NP. Specifically:
- There exists an oracle relative to which NP^{BQP} ⊄ BQP^{PH}, resolving a 2005 problem of Fortnow. As a corollary, there exists an oracle relative to which 𝖯 = NP but BQP ≠ QCMA. 
- Conversely, there exists an oracle relative to which BQP^{NP} ⊄ PH^{BQP}. 
- Relative to a random oracle, PP is not contained in the "QMA hierarchy" QMA^{QMA^{QMA^{⋯}}}. 
- Relative to a random oracle, Σ_{k+1}^𝖯 ⊄ BQP^{Σ_k^𝖯} for every k. 
- There exists an oracle relative to which BQP = P^#P and yet PH is infinite. (By contrast, relative to all oracles, if NP ⊆ BPP, then PH collapses.) 
- There exists an oracle relative to which 𝖯 = NP ≠ BQP = 𝖯^#P. 
To achieve these results, we build on the 2018 achievement by Raz and Tal of an oracle relative to which BQP ⊄ PH, and associated results about the Forrelation problem. We also introduce new tools that might be of independent interest. These include a "quantum-aware" version of the random restriction method, a concentration theorem for the block sensitivity of AC⁰ circuits, and a (provable) analogue of the Aaronson-Ambainis Conjecture for sparse oracles.

Subject Classification

ACM Subject Classification
  • Theory of computation → Quantum complexity theory
  • Theory of computation → Complexity classes
Keywords
  • BQP
  • Forrelation
  • oracle separations
  • Polynomial Hierarchy
  • query complexity

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