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arXiv:2608.06459

KPZ Superdiffusion of Local Correlators in Diffusive Random Quantum Circuits

Ewan McCulloch

quant-phcond-mat.dis-nncond-mat.stat-mechcond-mat.str-elSubmitted Aug 6, 2026• Announced Aug 10, 2026• 0 citations
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Abstract

We study the single-particle Green's function $G(x,t)=\langle σ^-_x(0)σ^+_0(t)\rangle$ in one-dimensional particle-number-conserving random unitary circuits coupled to an external bath. For fixed spacetime disorder, we argue that $G(x,t)$ is governed, in both the strong- and weak-noise limits, by directed waves in a random medium. We find Kardar-Parisi-Zhang (KPZ) scaling in the wandering statistics of the normalized spatial distribution $p(x,t)\propto |G(x,t)|^2$ and in the associated free energy. In particular, its center $\langle x(t)\rangle\equiv\sum_x x\, p(x,t)$ wanders on a length-scale $\mathcal{O}(t^{2/3})$, while sample-to-sample fluctuations of $-\log\sum_x |G(x,t)|^2$ scale as $t^{1/3}$. At weak noise $γ\ll 1$, the crossover to the strong-disorder fixed point occurs at a parametrically long time $\mathcal{O}(γ^{-3/2})$. These predictions are confirmed numerically using tensor-network simulations of the noisy operator dynamics in individual circuits at moderate noise, and of a phase-annealed proxy retaining hopping disorder at weak noise.

Comments
5 pages, 2 figures (with 2 pages End Matter and 11 pages Supplemental Material)

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