Why is probabilistic error cancellation expensive?

Question: Probabilistic Error Cancellation represents an ideal operation as a signed combination of experimentally implementable noisy operations. What produces its principal sampling overhead? A) The number of ancilla qubits needed to coherently store every noisy operation B) The requirement to perform complete state tomography after each circuit layer C) The variance amplification associated with sampling signed quasiprobability coefficients whose absolute values sum to more than one D) The need to increase circuit depth until the physical noise becomes negligible Correct: C Explanation: Because the inverse-noise representation generally uses negative or nonprobabilistic coefficients, circuits must be sampled according to absolute coefficient magnitudes and their signed outcomes recombined. The squared quasiprobability normalization can lead to substantial—and potentially exponential—sampling overhead. Topic: advanced quantum computing / probabilistic error cancellation / mitigation