Quantum Depth in the Random Oracle Model

Abstract: We give a comprehensive characterization of the computational power of shallow quantum circuits combined with classical computation. Specifically, for classes of search problems, we show that the following statements hold, relative to a random oracle: (a) $$\mathsf{BPP}^{{\mathsf{QNC}^{{\mathsf{BPP}}}}} \neq \mathsf{BQP}$$. This refutes Jozsa's conjecture [QIP 05] in the random oracle model. As a result, this gives the first instantiatable separation between the classes by replacing the oracle with a cryptographic hash function, yielding a resolution to one of Aaronson's ten semi-grand challenges in quantum computing. (b) $$\mathsf{BPP}^{{\mathsf{QNC}}} \nsubseteq \mathsf{QNC}<sup>{\mathsf{BPP}}$$ and $$\mathsf{QNC}^{{\mathsf{BPP}}} \nsubseteq \mathsf{BPP}<sup>{\mathsf{QNC}}$$. This shows that there is a subtle interplay between classical computation and shallow quantum computation. In fact, for the second separation, we establish that, for some problems, the ability to perform adaptive measurements in a single shallow quantum circuit, is more useful than the ability to perform polynomially many shallow quantum circuits without adaptive measurements. (c) There exists a 2-message proof of quantum depth protocol. Such a protocol allows a classical verifier to efficiently certify that a prover must be performing a computation of some minimum quantum depth. Our proof of quantum depth can be instantiated using the recent proof of quantumness construction by Yamakawa and Zhandry [STOC 22].