Local Unitary Equivalent Classes of Symmetric N-Qubit Mixed States

TL;DR

This paper introduces a new geometric classification method for symmetric N-qubit mixed states using Multiaxial Representation, extending entanglement classification beyond pure states and providing a comprehensive framework applicable to mixed states.

Contribution

The authors develop a novel classification scheme for symmetric N-qubit mixed states based on Multiaxial Representation, incorporating rank as a new parameter and generalizing pure state methods.

Findings

The classification scheme applies to both pure and mixed states.

Detailed analysis of GHZ states highlights differences from pure state classification.

The method effectively classifies symmetric two and three qubit states, including experimentally realizable states.

Abstract

Majorana Representation (MR) of symmetric $ N $-qubit pure states has been used successfully in entanglement classification. Generalization of this has been a long standing open problem due to the difficulties faced in the construction of a Majorana like geometric representation for symmetric mixed state. We have overcome this problem by developing a method of classifying local unitary (LU) equivalent classes of symmetric $N$-qubit mixed states based on the geometrical Multiaxial Representation (MAR) of the density matrix. In addition to the two parameters defined for the entanglement classification of the symmetric pure states based on MR, namely, diversity degree and degeneracy configuration, we show that another parameter called rank needs to be introduced for symmetric mixed state classification. Our scheme of classification is more general as it can be applied to both pure and mixed states. To bring out the similarities/ differences between the MR and MAR, $ N $-qubit GHZ state is taken up for a detailed study. We conclude that pure state classification based on MR is not a special case of our classification scheme based on MAR. We also give a recipe to identify the most general symmetric $N$-qubit pure separable states. The power of our method is demonstrated using several well known examples of symmetric two qubit pure and mixed states as well as three qubit pure states. Classification of uniaxial, Biaxial and triaxial symmetric two qubit mixed states which can be produced in the laboratory is studied in detail.