Classical sampling of random circuits with non-unital noise
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
Noise that scrambles qubits toward randomness makes deep quantum circuits easy to imitate classically. Noise that pushes qubits toward a fixed state, as energy loss does, behaves differently, and whether such noisy circuits can be imitated is open.
Precise statement
For random geometrically local circuits on $n$ qubits (Haar two-qubit gates, brickwork, depth $D$) with single-qubit amplitude damping of strength $\gamma > 0$ after each gate and no mid-circuit measurement, is there a $\operatorname{poly}(n, 1/\delta)$ classical algorithm that samples the output to total-variation error $\delta$ for all $D \ge D_0(\gamma)$, with $D_0$ independent of $n$? Local expectation values are classically computable because this noise limits the effective depth to $O(\log n)$ (Mele et al. 2024), but outputs do not anticoncentrate (Fefferman et al. 2024), so existing sampling arguments fail; for unital depolarizing noise of strength $p$, a 2026 preprint proves efficient sampling for $D \ge \Omega(p^{-1} \log p^{-1})$ in any geometrically local circuit (arXiv:2610.00548).
What would settle it
An efficient sampling algorithm with proof for the non-unital model above a noise-set depth, or a hardness argument for some depth regime growing with n.
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