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Polynomial inequalities for multipartite quantum systems in mixed states

Iurie Palii

Published Jun 15, 2018• 0 citations
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Abstract

Entanglement in a multipartite quantum system represents non-local correlations between the system parts. This phenomenon is not affected by the action of a symmetry group, which acts separately on each part of the multipartite composite system. The space of the invariants of the action is a natural domain of the definition for all possible measures of the entanglement. Allowed values of the invariants are restricted by a set of inequalities, which is analyzed in this paper. The inequalities ensue from the semipositivity and Hermicity of the density matrix for the system as well as from the orbit space description in terms of the invariant theory.

DOI
10.5281/zenodo.4019179

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Entanglement in a multipartite quantum system represents non-local correlations between the system parts. This phenomenon is not affected by the action of a symmetry group, which acts separately on each part of the multipartite composite system. The space of the invariants of the action is a natural domain of the definition for all possible measures of the entanglement. Allowed values of the invariants are restricted by a set of inequalities, which is analyzed in this paper. The inequalities ensue from the semipositivity and Hermicity of the density matrix for the system as well as from the orbit space description in terms of the invariant theory.
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We derive a classification and a measure of classical- and quantum-correlation of multipartite qubit, qutrit, and in general, nnn-level systems, in terms of SU(n)(n)(n) representations of density matrices. We compare the measure for the case of bipartite correlation with concurrence and the entropy of entanglement. The characterization of correlation is in terms of the number of nonzero singular values of the correlation matrix, but that of mixed state entanglement requires additional invariant parameters in the density matrix. For the bipartite qubit case, the condition for mixed state entanglement is written explicitly in terms of the invariant paramters in the density matrix. For identical particle systems we analyze the effects of exchange symmetry on classical and quantum correlation.
We discuss the entanglement properties of two qubits in terms of polynomial invariants of the adjoint action of SU(2) ⊕ SU(2) group on the space of density matrices P+\mathfrak{P}_ +P+​ . Since elements of P+\mathfrak{P}_ +P+​ are Hermitian, non-negative fourth-order matrices with unit trace, the space of density matrices represents a semi-algebraic subset, P+∈R15\mathfrak{P}_ + \in \mathbb{R}^{15}P+​∈R15 . We define P+\mathfrak{P}_ +P+​ explicitly with the aid of polynomial inequalities in the Casimir operators of the enveloping algebra of SU(4) group. Using this result the optimal integrity basis for polynomial SU(2) ⊕ SU(2) invariants is proposed and the well-known Peres-Horodecki separability criterion for 2-qubit density matrices is given in the form of polynomial inequalities in three SU(4) Casimir invariants and two SU(2) ⊕ SU(2) scalars; namely, determinants of the so-called correlation and the Schlienz-Mahler entanglement matrices.
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In the present paper few steps are undertaken towards the description of the qubit-qutrit pair - quantum bipartite system composed of two and three level subsystems. The computational difficulties with the construction of the local unitary polynomial invariants are discussed. Calculations of the Molien functions and Poincare series for the qubit-qubit and qubit-qutrit local unitary invariants are outlined and compared with the known results. The requirement of positive semi-definiteness of the density operator is formulated explicitly as a set of inequalities in five Casimir invariants of the algebra su(6).