Invariant Measures and Weak-Magic-Injection Asymptotics in Random Monitored Quantum Circuits

Abstract

Monitored quantum circuits combine scrambling with measurement-conditioned state updates, while non-Clifford perturbations inject magic into otherwise stabilizer-compatible dynamics. Rigorous results on stationary magic and its weak-injection asymptotics remain limited even for finite-dimensional Clifford-based monitored models. We study an \(N\)-qudit process of prime local dimension \(d\). Each cycle draws a fresh uniform global Clifford unitary, applies a local weak non-Clifford rotation and a projective measurement on one qudit, and then returns to the inverse Clifford frame. For every fixed injection strength, we prove that the induced pure-state Markov chain has a unique invariant probability measure and attracts every initial law geometrically in Wasserstein distance. At zero injection, the invariant law is supported on the finite stabilizer layer. After rescaling transverse deviations from this layer, the resulting blown-up stationary laws converge weakly to the invariant law of an affine tangent recursion. Combining this tangent law with Poisson representations and the first nonzero local resource germs determines the sharp vanishing rates of stationary magic. For every fixed \(N\ge2\), odd-prime Gross--Wigner mana admits a linear expansion with a strictly positive coefficient, whereas qubit \(2\)-stabilizer Rényi entropy admits a quadratic expansion with a strictly positive coefficient. For \(N=1\), both stationary resource averages vanish identically for all injection strengths. The distinct orders arise from the different local resource geometries, together with quadratic-order zero-reference branch contributions in qubit case.