Why can Pauli twirling hide coherent structure?

Question: A coherent over-rotation channel is Pauli-twirled before benchmarking. Which statement is most accurate? A) Twirling can convert coherent error structure into an effective Pauli-stochastic description, but it may obscure worst-case coherent behaviour B) Twirling reconstructs the exact coherent Hamiltonian generator of the noise C) Twirling always lowers the physical error rate of the hardware gate D) Twirling makes the channel unitary by removing all stochastic components Correct: A Explanation: Pauli twirling can simplify a noise channel into a Pauli-stochastic form for analysis or mitigation. However, the twirled description may not retain all operational details of the original coherent error, especially worst-case accumulation behaviour. Topic: advanced quantum computing / Pauli twirling / coherent error