One of the most basic conjectures in quantum complexity theory states that every t-query quantum algorithm can be simulated on most inputs by a poly(t)-query classical algorithm. If true, this would provide broad justification for the need for structure in quantum speedups. We settle this conjecture for parallel quantum algorithms, showing that every t-query d-round quantum algorithm can be simulated on most inputs with tO(d2)
Nearby in the stack
classical queries. This suggests that for unstructured problems, superpolynomial speedups would require quantum circuits of superconstant depth, and exponential speedups would further require polynomial depth. In contrast, most known speedups for structured problems are achieved by highly parallel, low-depth algorithms. Our techniques also carry new implications for the status of
BPP
vs.
BQP
relative to a random oracle, a similarly longstanding problem.