Oracle Separations for the Quantum-Classical Polynomial Hierarchy

TL;DR

This paper demonstrates that the quantum-classical polynomial hierarchy is infinite relative to a random oracle and introduces a new quantum switching lemma that bounds quantum query complexity under random restrictions.

Contribution

It proves the infinitude of QCPH relative to a random oracle and develops a novel quantum switching lemma applicable to quantum query complexity and polynomial degree.

Findings

QCPH is infinite relative to a random oracle

Higher levels of PH are not contained in lower levels of QCPH relative to a random oracle

Introduces a new quantum switching lemma for low-depth alternating circuits

Abstract

We study the quantum-classical polynomial hierarchy, QCPH, which is the class of languages solvable by a constant number of alternating classical quantifiers followed by a quantum verifier. Our main result is that QCPH is infinite relative to a random oracle (previously, this was not even known relative to any oracle). We further prove that higher levels of PH are not contained in lower levels of QCPH relative to a random oracle; this is a strengthening of the somewhat recent result that PH is infinite relative to a random oracle (Rossman, Servedio, and Tan 2016). The oracle separation requires lower bounding a certain type of low-depth alternating circuit with some quantum gates. To establish this, we give a new switching lemma for quantum algorithms which may be of independent interest. Our lemma says that for any $d$, if we apply a random restriction to a function $f$ with quantum query complexity $\mathrm{Q}(f)\le n^{1/3}$, the restricted function becomes exponentially close (in terms of $d$) to a depth-$d$ decision tree. Our switching lemma works even in a "worst-case" sense, in that only the indices to be restricted are random; the values they are restricted to are chosen adversarially. Moreover, the switching lemma also works for polynomial degree in place of quantum query complexity.