# Numerical Simulation of Critical Dissipative Non-Equilibrium Quantum Systems with an Absorbing State

## TL;DR

This paper explores the use of tensor network methods, specifically matrix product states and time-evolving block decimation, to simulate the critical dynamics of a non-equilibrium quantum system with an absorbing state, revealing insights into its universality class.

## Contribution

It demonstrates the effectiveness of the Heisenberg picture and quantum trajectories in improving simulation accuracy for the quantum contact process at criticality.

## Key findings

- Heisenberg picture improves simulation accuracy over Schrödinger approach
- Quantum trajectories can simulate longer timescales
- Quantum contact process does not belong to directed percolation universality class

## Abstract

The simulation of out-of-equilibrium dissipative quantum many body systems is a problem of fundamental interest to a number of fields in physics, ranging from condensed matter to cosmology. For unitary systems, tensor network methods have proved successful and extending these to open systems is a natural avenue for study. In particular, an important question concerns the possibility of approximating the critical dynamics of non-equilibrium systems with tensor networks. Here, we investigate this by performing numerical simulations of a paradigmatic quantum non-equilibrium system with an absorbing state: the quantum contact process. We consider the application of matrix product states and the time-evolving block decimation algorithm to simulate the time-evolution of the quantum contact process at criticality. In the Lindblad formalism, we find that the Heisenberg picture can be used to improve the accuracy of simulations over the Schr\"{o}dinger approach, which can be understood by considering the evolution of operator-space entanglement. Furthermore, we also consider a quantum trajectories approach, which we find can reproduce the expected universal behaviour of key observables for a significantly longer time than direct simulation of the average state. These improved results provide further evidence that the universality class of the quantum contact process is not directed percolation, which is the class of the classical contact process.