Anomalous Directed Percolation on a Dynamic Network using Rydberg Facilitation

TL;DR

This paper investigates how Rydberg facilitation in atomic gases can exhibit various universality classes of non-equilibrium phase transitions, including anomalous directed percolation, by tuning network dynamics and long-range excitation processes.

Contribution

It demonstrates that the universality class of Rydberg facilitation transitions can be tuned between directed percolation, mean-field, and anomalous classes using simulations and machine learning.

Findings

Identification of anomalous directed percolation universality class.

Prediction of critical exponents varying with atomic motion.

Explanation of experimental critical exponents observed in ultra-cold gases.

Abstract

The facilitation of Rydberg excitations in a gas of atoms provides an ideal model system to study epidemic evolution on (dynamic) networks and self organization of complex systems to the critical point of a non-equilibrium phase transition. Using Monte-Carlo simulations and a machine learning algorithm we show that the universality class of this phase transition can be tuned. The classes include directed percolation (DP), the most common class in short-range spreading models, and mean-field (MF) behavior, but also different types of anomalous directed percolation (ADP), characterized by rare long-range excitation processes. In a frozen gas, ground state atoms that can facilitate each other form a static network, for which we predict DP universality. Atomic motion then turns the network into a dynamic one with long-range (Levy-flight type) excitations. This leads to continuously varying critical exponents corresponding to the ADP universality class, eventually reaching MF behavior. These findings also explain the recently observed critical exponent of Rydberg facilitation in an ultra-cold gas experiment [Helmrich et al., Nature 577, 481 (2020)], which was in between DP and MF values.