Dirichlet-Constrained Random Operators and Quenched Reflection Quantum Walks: Tree Spectra and Poisson Criticality at High Degree

Source abstract

We study spectral limits of random operators whose incident edge weights are coupled by local normalization, and the reflection quantum walks associated with them. At each vertex of a $d$-regular graph, an independent Dirichlet probability vector distributes unit mass among outgoing directions. At fixed degree, the empirical spectral measures of the associated self-adjoint operators converge almost surely on locally tree-like graph sequences. We give a closed-walk formula for every moment of the limiting tree law. Our main result identifies two distinct high-degree scales. After rescaling the deterministic expected root spectral law by $\sqrt d$, a weak limit exists if and only if $dα_d^2$ converges in $[0,\infty]$, where $α_d$ is the Dirichlet parameter. The limits range from a point mass at zero through non-semicircular critical laws to the semicircle law. The critical laws admit a Poisson-tree representation with an explicit Bessel intensity. The extended family, including both endpoint laws, provides a first-Wasserstein approximation uniform over positive Dirichlet parameters. In the regime $dα_d^2\to0$, the rescaled law converges weakly to a point mass although its second moment remains one. Weighting spectral mass by its contribution to this moment and rescaling again reveals a different family of limits, classified by $dα_d$. These limits have explicit Beta-product descriptions. A quantitative comparison transfers the variance-scale classification to the centered and rescaled inherited eigenphases of the quantum walk.