A sharp interaction-degree threshold for simulating QAOA

Abstract

We identify a sharp interaction-degree threshold for the classical simulation of QAOA with $2$-local cost functions. At degree~$3$, classical sampling from depth-$1$ QAOA, even within multiplicative error $2^{n^{s}}$ for any fixed $s < 1$, would collapse the polynomial hierarchy to its third level. At degree $2$, exact classical sampling from depth-$p$ QAOA on $n$ qubits runs in time $n^{O(1)}$ whenever $p = O(\log n)$. The hard degree-$3$ instances have trivially optimizable cost functions, so sampling hardness does not by itself imply a quantum optimization advantage.