Reproduction of Zero Noise Extrapolation: Temme, Bravyi & Gambetta (PRL 119, 180509, 2017)

Full reproduction of the Zero Noise Extrapolation (ZNE) method from Temme, Bravyi & Gambetta (Physical Review Letters 119, 180509, 2017). Implemented from scratch using NumPy density matrix simulation with depolarizing noise channels. Covers: (1) gate folding for noise amplification U → UU†U, (2) Richardson extrapolation for zero-noise estimation, and (3) reproduction of Figures 1–3 from the original paper. Key finding documented: Richardson extrapolation reliably reduces error for base noise λ ≤ 0.02 but overshoots at λ ≥ 0.05 when amplified noise (5λ = 0.25) violates the perturbative Taylor expansion — consistent with the paper's stated NISQ hardware regime of λ ~ 0.001–0.01. All source code: https://github.com/aashiq-parinda/quantum-zne-reproduction21/21 unit tests passing.