Nonlocality and entanglement in qubit systems

TL;DR

This paper investigates the relationship between nonlocality and entanglement in multi-qubit systems, identifying maximally nonlocal states, analyzing their properties, and extending the study from two to four qubits with applications in quantum information.

Contribution

It provides a comprehensive analysis of nonlocality in multi-qubit systems, including optimization of Bell inequality violations and extensions to larger qubit systems, highlighting differences from entanglement.

Findings

Maximally violating Bell inequalities for two-qubit states identified.

Closed-form expressions for nonlocality of pure and mixed states derived.

Extension of nonlocality analysis to three and four-qubit systems achieved.

Abstract

Nonlocality and quantum entanglement constitute two special aspects of the quantum correlations existing in quantum systems, which are of paramount importance in quantum-information theory. Traditionally, they have been regarded as identical (equivalent, in fact, for pure two qubit states, that is, {\it Gisin's Theorem}), yet they constitute different resources. Describing nonlocality by means of the violation of several Bell inequalities, we obtain by direct optimization those states of two qubits that maximally violate a Bell inequality, in terms of their degree of mixture as measured by either their participation ratio $R=1/Tr(\rho^2)$ or their maximum eigenvalue $\lambda_{max}$. This optimum value is obtained as well, which coincides with previous results. Comparison with entanglement is performed too. An example of an application is given in the XY model. In this novel approximation, we also concentrate on the nonlocality for linear combinations of pure states of two qubits, providing a closed form for their maximal nonlocality measure. The case of Bell diagonal mixed states of two qubits is also extensively studied. Special attention concerning the connection between nonlocality and entanglement for mixed states of two qubits is paid to the so called maximally entangled mixed states. Additional aspects for the case of two qubits are also described in detail. Since we deal with qubit systems, we will perform an analogous study for three qubits, employing similar tools. Relation between distillability and nonlocality is explored quantitatively for the whole space of states of three qubits. We finally extend our analysis to four qubit systems, where nonlocality for generalized Greenberger-Horne-Zeilinger states of arbitrary number of parties is computed.