The unitary $ \mathrm{U}(d)$-equivalence relation between elements of the space $\mathfrak{P}_+\,$ of mixed states of $d$ -dimensional quantum system defines the orbit space $ \mathfrak{P}_+/ \mathrm{U}(d)\,$ and provides its description in terms the ring $\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,$ of $\mathrm{U}(d)$ -invariant polynomials. We prove that the semi-algebraic structure of $ \mathfrak{P}_+/ \mathrm{U}(d)\, $ is determined completely by two basic properties of density matrices, their semi-positivity and Hermicity. Particularly, it is shown that the Processi-Schwarz inequalities in elements of integrity basis for $\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,$ defining the orbit space, are identically satisfied for all elements of $\mathfrak{P}_+$ .The unitary $ \mathrm{U}(d)$-equivalence relation between elements of the space $\mathfrak{P}_+\,$ of mixed states of $d$-dimensional quantum system defines the orbit space $ \mathfrak{P}_+/ \mathrm{U}(d)\,$ and provides its description in terms the ring $\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,$ of $\mathrm{U}(d)$-invariant polynomials. We prove that the semi-algebraic structure of $ \mathfrak{P}_+/ \mathrm{U}(d)\, $ is determined completely by two basic properties of density matrices, their semi-positivity and Hermicity. Particularly, it is shown that the Processi-Schwarz inequalities in elements of integrity basis for $\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,$ defining the orbit space, are identically satisfied for all elements of $\mathfrak{P}_+$.

arXiv:1311.4649Nov 19, 20130 citations

On the Stratifications of 2-Qubits X-State Space

A. Khvedelidze, Astghik Torosyan

The 7-dimensional family ℬX of the so-called mixed X-states of 2-qubits is considered. Two versions of stratifications of ℬX , i.e., its decomposition into strata according to orbit types of the adjoint actions of two groups, are described. The first stratification is due to the action of global unitary group GX ⊂ S U(4), while the second one corresponds to the action of the local unitary group LGX ⊂ GX . The equations and in-equalities in the invariants of the corresponding groups, determining each stratification component, are given.The 7-dimensional family ℬX of the so-called mixed X-states of 2-qubits is considered. Two versions of stratifications of ℬX , i.e., its decomposition into strata according to orbit types of the adjoint actions of two groups, are described. The first stratification is due to the action of global unitary group GX ⊂ S U(4), while the second one corresponds to the action of the local unitary group LGX ⊂ GX . The equations and in-equalities in the invariants of the corresponding groups, determining each stratification component, are given.

arXiv:1708.07438EPJ Web of ConferencesJan 1, 20180 citations

Rational local unitary invariants of symmetrically mixed states of two qubits

Luca Candelori, Vladimir Chernyak, John R. Klein +1 more

arXiv:2304.13555Advances in Applied MathematicsSep 18, 20232 citations

Unitary combinations of formalized classes in qubit space

Nikolay Raychev

International Journal of Scientific and Engineering ResearchApr 25, 20152 citations

Witness of mixed separable states useful for entanglement creation

Nirman Ganguly, Jyotishman Chatterjee, A. S. Majumdar

Absolutely separable states form a special subset of the set of all separable states, as they remain separable under any global unitary transformation unlike other separable states. We consider the set of absolutely separable bipartite states and show that it is convex and compact in any arbitrary dimensional Hilbert space. Through a generic approach of construction of suitable Hermitian operators we prove the completeness of the separation axiom for identifying any separable state that is not absolutely separable. We demonstrate the action of such witness operators in different qudit systems. Examples of mixed separable systems are provided, pointing out the utility of the witness in entanglement creation using quantum gates. Decomposition of witnesses in terms of spin operators or photon polarizations facilitates their measurability for qubit states.Absolutely separable states form a special subset of the set of all separable states, as they remain separable under any global unitary transformation unlike other separable states. We consider the set of absolutely separable bipartite states and show that it is convex and compact in any arbitrary dimensional Hilbert space. Through a generic approach of construction of suitable Hermitian operators we prove the completeness of the separation axiom for identifying any separable state that is not absolutely separable. We demonstrate the action of such witness operators in different qudit systems. Examples of mixed separable systems are provided, pointing out the utility of the witness in entanglement creation using quantum gates. Decomposition of witnesses in terms of spin operators or photon polarizations facilitates their measurability for qubit states.

arXiv:1401.5324Physical Review AMay 8, 201437 citations

The unitary $ \mathrm{U}(d)$-equivalence relation between elements of the space

\mathfrak{P}_+\,

of mixed states of

d

-dimensional quantum system defines the orbit space $ \mathfrak{P}_+/ \mathrm{U}(d)\,$ and provides its description in terms the ring

\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,

\mathrm{U}(d)

-invariant polynomials. We prove that the semi-algebraic structure of $ \mathfrak{P}_+/ \mathrm{U}(d)\, $ is determined completely by two basic properties of density matrices, their semi-positivity and Hermicity. Particularly, it is shown that the Processi-Schwarz inequalities in elements of integrity basis for

\mathbb{R}[\mathfrak{P}_+]^{\mathrm{U}(d)}\,

defining the orbit space, are identically satisfied for all elements of

\mathfrak{P}_+