Shannon entropy of the measurement record at measurement-dominated criticality and RG flow: A c-theorem for effective central charge and a g-theorem for effective boundary entropy

TL;DR

This paper proves that certain effective entropy measures decrease under RG flow in measurement-dominated critical systems, establishing c- and g-theorems analogous to those in conformal field theory, with implications for classical and quantum criticality.

Contribution

It introduces non-perturbative theorems showing the decrease of effective central charge and boundary entropy under RG flow in measurement-induced criticality, extending c- and g-theorems.

Findings

Effective central charge $c_{eff}$ is less than the unmeasured system's central charge.

Effective boundary entropy $ ext{ln} g_{eff}$ decreases under RG flow.

Potential increase of $c_{eff}$ and $g_{eff}$ in disordered classical systems at $R ightarrow0$.

Abstract

We present two theorems demonstrating non-perturbatively the decrease under relevant renormalization group (RG) flow of two quantities, $c_{\text{eff}}$ and $g_{\text{eff}}$ characterizing, respectively, the universal information content of the Shannon entropy of the measurement record for two different types of measurement-dominated criticality. First, we demonstrate the decrease of the "effective central charge" $c_{\text{eff}}$ of $2D$ replica field theories in the $R\rightarrow1$ replica limit that govern the long-distance physics of weakly monitored $2D$ classical critical systems (Baysian inference problems) studied recently in the literature [arXiv:2504.01264; arXiv:2504.12385; arXiv:2504.08888]. In particular, we show that $c_{\text{eff}}$ is $\textit{less}$ than the central charge $c$ of the unmeasured critical system. We refer to this result as the "$c$-effective theorem''. In addition, we present an analogous "$g$-effective theorem" demonstrating the decrease under RG flow of the effective "Affleck-Ludwig'' boundary entropy $\ln g_\text{eff}$, quantifying a corresponding contribution to the Shannon entropy for analogous $2D$ $\textit{defect}$ replica field theories in the $R\rightarrow1$ replica limit, which govern the long-distance physics in the problem of performing weak $\textit{quantum}$ measurements on one-dimensional quantum critical ground states. Lastly, we discuss a possible consequence of our theorems for classical systems with generic uncorrelated impurity-type quenched disorder, according to which, under a certain assumption, and as opposed to problems with measurement-induced randomness, the corresponding universal quantities $c_{\text{eff}}^{(R\rightarrow0)}$ and $g_{\text{eff}}^{(R\rightarrow0)}$ in the $R\rightarrow0$ replica limit would $\textit{increase}$ under RG flow.