# A toy model for time evolving QFT on a lattice with controllable chaos

## TL;DR

This paper introduces a quantum lattice model with controllable chaos, analytically computes Lyapunov exponents, and explores entanglement dynamics, offering insights into quantum chaos, information spreading, and potential quantum computing implementations.

## Contribution

It presents a toy model for quantum fields on a lattice with analytically tractable chaos and entanglement properties, including realizations on quantum computers.

## Key findings

- Lyapunov exponents can be computed analytically at large N.
- Entanglement rates are controlled by log(N) for certain states.
- The dynamics exhibit self-similar and peculiar behaviors.

## Abstract

A class of models with a dynamics of generalized quantum cat maps on a product of quantum tori is described. These tori are defined by an algebra of clock-shift matrices of dimension $N$. The dynamics is such that the Lyapunov exponents can be computed analytically at large $N$. Some of these systems can be thought of as a toy model for quantum fields on a lattice under a time evolution with nearest neighbor interactions, resembling a quantum version of a cellular automaton. The dynamics of entangling is studied for initial product states. Some of these entangle at rates determined by Lyapunov exponents of the system at large $N$ when the initial states are gaussian. For other classes of states, entanglement between two regions can be computed analytically: it is found that entanglement rates are controlled by $\log(N)$. Some of these setups can be realized on quantum computers with CNOT quantum gates. This is analyzed in detail where we find that the dynamics has a self-similar behavior and various peculiar behaviors. This dynamics can be interpreted in a particular basis as a machine that broadcasts classical messages in one direction and that produces over time a generalized GHZ state with the receiving region once we consider superpositions of such messages. With the appropriate choice of quantum vacuum on the receiving end of the system one can stop the message from leaving the broadcast area.