Complexity of two-level systems

TL;DR

This paper investigates the complexity of two-level quantum systems using correlational and thermal entropy, revealing conditions for maximal complexity and providing insights into system behavior through entropic measures.

Contribution

It introduces a basis-independent entropic complexity measure for two-level systems and analyzes its behavior under various disorder and thermal conditions.

Findings

Maximal complexity occurs at specific disorder strengths.

Entropic complexity provides deeper understanding of system dynamics.

Analysis applies to spins, qubits, and magnetic systems.

Abstract

Complexity of two-level systems, e.g. spins, qubits, magnetic moments etc, are analysed based on the so-called correlational entropy in the case of pure quantum systems and the thermal entropy in case of thermal equilibrium that are suitable quantities essentially free from basis dependence. The complexity is defined as the difference between the Shannon-entropy and the second order R\'enyi-entropy, where the latter is connected to the traditional participation measure or purity. It is shown that the system attains maximal complexity for special choice of control parameters, i.e. strength of disorder either in the presence of noise of the energy states or the presence of disorder in the off diagonal coupling. It is shown that such a noise or disorder dependence provides a basis free analysis and gives meaningful insights. We also look at similar entropic complexity of spins in thermal equilibrium for a paramagnet at finite temperature, $T$ and magnetic field $B$, as well as the case of an Ising model in the mean-field approximation. As a result all examples provide important evidence that the investigation of the entropic complexity parameters help to get deeper understanding in the behavior of these systems.