Fast Mixing for Low-Temperature Potts Models via Poisson Trees

2026-07-31

AI Summary

This work investigates the slow mixing of Markov chains for the Potts model on sparse random graphs $G(n,d/n)$ at low temperatures, where the ordered phase dominates. Focusing on the local limit—a Poisson Galton–Watson tree with monochromatic boundary conditions—the authors extend the rapid mixing results previously established for regular trees to the irregular-degree setting of Poisson trees. By developing a refined analytical framework based on adaptive block decomposition, decay of correlations estimates, and functional inequalities, they establish near-linear mixing time for Glauber dynamics in the low-temperature regime. This analysis yields the first near-linear-time approximate sampling algorithm for the Potts model on $G(n,d/n)$ that is valid across all temperatures.

Abstract

The $q$-state ferromagnetic Potts model on a graph $G$ is a probability distribution on all $q$-colourings of $G$ that favours many monochromatic edges. Approximate sampling from the Potts model is a central problem in the study of spin systems on sparse graphs, especially in the low-temperature regime, where the model strongly favours ordered configurations, often creating bottlenecks that make Markov-chain sampling inefficient or difficult to analyse. We focus on the sparse random graph $G(n,d/n)$. The local neighbourhoods of $G(n,d/n)$ are tree-like, but the relevant underlying graph is a Poisson Galton-Watson tree. This motivates the study of Glauber dynamics for the low-temperature Potts model on such trees with monochromatic boundary conditions. The Poisson setting introduces difficulties absent from the regular case: degrees fluctuate, long induced paths may appear, and branches can terminate before reaching the boundary. As a result, the effect of the monochromatic boundary at the leaves is much less uniform. Our main result shows near-linear mixing for the Glauber dynamics on Poisson trees with monochromatic boundary conditions. This extends the corresponding regular-tree results of Martinelli, Sinclair, and Weitz (SODA 2004) and of Blanca, Chen, Stefankovič, and Vigoda (RANDOM 2021) to the irregular trees arising from sparse random graphs. Our proof introduces an adaptive block decomposition of the tree, built around regions containing large regular subtrees, and combines it with correlation-decay estimates and functional-inequality arguments. We also obtain a near-linear-time approximate sampling algorithm for the Potts model on $G(n,d/n)$ at all temperatures, speeding up the best previous algorithm of Galanis, Goldberg, and Smolarova (ICALP 2025). The main new ingredient is a refined analysis of the low-temperature regime, building on the Poisson tree result.

Problem

• Potts model
• low-temperature
• approximate sampling
• sparse random graphs
• mixing time

Innovation

• Poisson trees
• Glauber dynamics
• near-linear mixing
• adaptive block decomposition
• Potts model