Evaluating QAOA expectation values can be as hard as counting optimal solutions
This work investigates the computational complexity of evaluating expectation values in the Quantum Approximate Optimization Algorithm (QAOA) for the MaxCut problem. By employing deterministic polynomial-time Turing reductions, Laurent polynomial analysis, and #P-hardness proof techniques, it establishes that for circuit depth \( p \geq 2 \), computing exact or exponentially precise expectation values—and their gradients and Hessians—is #P-hard, even when restricted to a single two-body correlation term or a constrained parameter set. This result demonstrates that the complexity transition from \( p = 1 \) to \( p \geq 2 \) in QAOA transcends mere NP-hardness, ascending to the #P level associated with counting optimal solutions. Moreover, the reduction simultaneously recovers both the maximum cut value and the number of optimal solutions, thereby revealing a fundamental computational barrier inherent to this task.