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arXiv:quant-ph/0612064

Concurrence of Lorentz-positive maps

Roland Hildebrand

Published Dec 8, 2006• 3 citations
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Abstract

Let L_n be the n-dimensional Lorentz cone. A linear map M from R^m to R^n is called Lorentz-positive if M[L_m] is contained in L_n. We extend the notion of concurrence, which was initially introduced to quantify the entanglement of bipartite density matrices, to Lorentz-positive maps and provide an explicite formula for it. This allows us to obtain formulae for the concurrence of arbitrary positive operators taking 2 x 2 complex hermitian matrices as input and consequently of arbitrary bipartite density matrices of rank 2. Namely, let P: H(2) \to H(d) be a positive operator, and let λ_1,...,λ_4 be the generalized eigenvalues of the pencil σ_2(P(X)) - λdet X, in decreasing order, where σ_2 is the second symmetric function of the spectrum. Then the concurrence is given by the expression C(P;X) = 2\sqrt{σ_2(P(X)) - λ_2 det X}. As an application, we compute the concurrences of the density matrices of all graphs with 2 edges. Similar results apply for a function which we call I-fidelity, with the second largest generalized eigenvalue λ_2 replaced by the smallest eigenvalue λ_4.

DOI
10.48550/arxiv.quant-ph/0612064

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Let L_n be the n-dimensional Lorentz cone. A linear map M from R^m to R^n is called Lorentz-positive if M[L_m] is contained in L_n. We extend the notion of concurrence, which was initially introduced to quantify the entanglement of bipartite density matrices, to Lorentz-positive maps and provide an explicite formula for it. This allows us to obtain formulae for the concurrence of arbitrary positive operators taking 2 x 2 complex hermitian matrices as input and consequently of arbitrary bipartite density matrices of rank 2. Namely, let P: H(2) \to H(d) be a positive operator, and let λ_1,...,λ_4 be the generalized eigenvalues of the pencil σ_2(P(X)) - λdet X, in decreasing order, where σ_2 is the second symmetric function of the spectrum. Then the concurrence is given by the expression C(P;X) = 2\sqrt{σ_2(P(X)) - λ_2 det X}. As an application, we compute the concurrences of the density matrices of all graphs with 2 edges. Similar results apply for a function which we call I-fidelity, with the second largest generalized eigenvalue λ_2 replaced by the smallest eigenvalue λ_4.
Concurrence is a widely used entanglement measure of bipartite mixed states. We propose to consider the concurrence as being defined by a quadratic form on the space of self-adjoint operators. The square root of this form determines the values of the concurrence on the pure states, while the values on the mixed states are obtained as the largest convex extension, the convex roof. This viewpoint admits a generalization. Namely, the space of self-adjoint operators and the convex cone of positive semidefinite operators contained therein can be replaced by an arbitrary real vector space containing a convex cone. Then the concurrence is determined on the extreme rays of the cone by the square root of a quadratic form, and on the rest of the cone by the convex roof. We compute this generalized concurrence in the case when the cone is a second order cone. This enables us to compute the concurrence of arbitrary bipartite mixed states of rank 2. As an application, we compute the concurrences of the density matrices of all graphs with two edges or with three edges forming a triangle. We also consider the problem of maximizing the concurrence on the set of mixed states having a fixed spectrum.
Let and let be the n-dimensional second-order cone, or Lorentz cone. A linear map M from to is called positive if M[Lm]⊂Ln . The set of positive maps forms a convex cone, the positive cone. Its dual cone, the separable cone, is the convex hull of tensor products x⊗y, where x∈Ln , y∈Lm . The structure of the positive and the separable cones is the subject of the present contribution. We study the action of the automorphism group of the positive cone and provide a complete classification of its orbits by constructing explicitly canonical forms. This enables us to identify and to classify the extreme rays of the positive cone, as well as their dual counterparts, the largest faces of the separable cone. We also characterize positive maps which are scalable to doubly stochastic ones.
Linear maps between finite-dimensional ordered vector spaces with orders induced by proper cones C A and C B are called entanglement breaking if their partial application sends the maximal tensor product K ⊗ max C A into the minimal tensor product K ⊗ min C B for any proper cone K . We study the larger class of Lorentz-entanglement breaking maps where K is restricted to be a Lorentz cone of any dimension, i.e., any cone over a Euclidean ball. This class of maps appeared recently in the study of asymptotic entanglement annihilation and it is dual to the linear maps factoring through Lorentz cones. Our main results establish connections between these classes of maps and operator ideals studied in the theory of Banach spaces. For operators u : X → Y between finite-dimensional normed spaces X and Y we consider so-called central maps which are positive with respect to the cones C A = C X and C B = C Y . We show how to characterize when such a map factors through a Lorentz cone and when it is Lorentz-entanglement breaking by using the Hilbert-space factorization norm γ 2 and its dual ⁎ γ 2 ⁎ . We also study the class of Lorentz-entanglement annihilating maps whose local application sends the Lorentzian tensor product C A ⊗ L C A into the minimal tensor product C B ⊗ min C B . When C A is a cone over a finite-dimensional normed space and C B is a Lorentz cone itself, the central maps of this kind can be characterized by the 2-summing norm π 2 . Finally, we prove interesting connections between these classes of maps for general cones, and we identify examples with particular properties, e.g., cones with an analogue of the 2-summing property.
Explicit separability of general two qubits density matrices is related to Lorentz transformations. We use the 4-dimensional form [Formula: see text] of the Hilbert–Schmidt (HS) decomposition of the density matrix. For the generic case, in which Lorentz transformations diagonalize [Formula: see text] into [Formula: see text], we give relations between the [Formula: see text] and the [Formula: see text]. In particular, we consider two cases: (a) Two qubits density matrices with one pair of linear terms in the HS decomposition. (b) Two qubits density matrices with two or three symmetric pairs of linear terms. Some of the theoretical results are demonstrated by numerical calculations. The four non-generic cases (which may be reduced to case (a) are analyzed and the non-generic property is related explicitly to Lorentz velocity [Formula: see text] which is not reachable physically.
Abstract Every multipartite entangled quantum state becomes fully separable after an entanglement breaking quantum channel acted locally on each of its subsystems. Whether there are other quantum channels with this property has been an open problem with important implications for entanglement theory (e.g., for the distillation problem and the PPT squared conjecture). We cast this problem in the general setting of proper convex cones in finite-dimensional vector spaces. The max-entanglement annihilating maps transform thek-fold maximal tensor product of a cone C1{\textsf{C}}_1C1​ C1 into thek-fold minimal tensor product of a cone C2{\textsf{C}}_2C2​ C2 , and the pair (C1,C2)({\textsf{C}}_1,{\textsf{C}}_2)(C1​,C2​) (C1,C2) is called resilient if all max-entanglement annihilating maps are entanglement breaking. Our main result is that (C1,C2)({\textsf{C}}_1,{\textsf{C}}_2)(C1​,C2​) (C1,C2) is resilient if either C1{\textsf{C}}_1C1​ C1 or C2{\textsf{C}}_2C2​ C2 is a Lorentz cone. Our proof exploits the symmetries of the Lorentz cones and applies two constructions resembling protocols for entanglement distillation: As a warm-up, we use the multiplication tensors of real composition algebras to construct a finite family of generalized distillation protocols for Lorentz cones, containing the distillation protocol for entangled qubit states by Bennett et al. (Phys Rev Lett 76(5):722, 1996) as a special case. Then, we construct an infinite family of protocols using solutions to the Hurwitz matrix equations. After proving these results, we focus on maps between cones of positive semidefinite matrices, where we derive necessary conditions for max-entanglement annihilation similar to the reduction criterion in entanglement distillation. Finally, we apply results from the theory of Banach space tensor norms to show that the Lorentz cones are the only cones with a symmetric base for which a certain stronger version of the resilience property is satisfied.
A pair of proper cones (C1,C2)(\mathsf{C}_1,\mathsf{C}_2)(C1​,C2​) is said to have the Lorentz factorization property (LFP) if every (C1,C2)(\mathsf{C}_1,\mathsf{C}_2)(C1​,C2​)-positive map factors through a direct sum of Lorentzian cones, i.e., cones over Euclidean balls. Clearly, (C1,C2)(\mathsf{C}_1,\mathsf{C}_2)(C1​,C2​) has the LFP if either C1\mathsf{C}_1C1​ or C2\mathsf{C}_2C2​ is a direct sum of Lorentzian cones, and our main goal is to find other examples. We show that such examples cannot be found for pairs (C1,C2)(\mathsf{C}_1,\mathsf{C}_2)(C1​,C2​) where C1=C2\mathsf{C}_1=\mathsf{C}_2C1​=C2​, or in the case where both C1\mathsf{C}_1C1​ and C2\mathsf{C}_2C2​ are polyhedral. We also focus on the case where C1=C□\mathsf{C}_1=\mathsf{C}_\squareC1​=C□​ is the square-based cone in R3\mathbf{R}^3R3. Here, we show that (C□,C)(\mathsf{C}_\square,\mathsf{C})(C□​,C) has the LFP whenever C\mathsf{C}C is a symmetric cone, i.e., a direct sum of Lorentz cones, cones of positive semidefinite matrices over the real numbers, complex numbers or quaternions, and the cone of 3×33\times 33×3 positive semidefinite matrices over the octonions. We leave open the question whether there are more examples, but we show that this list cannot be extended by any strictly convex cone C\mathsf{C}C or for a cone C\mathsf{C}C with dim(C)≤5\text{dim}(\mathsf{C})\leq 5dim(C)≤5. Finally, we discuss an application to a problem in quantum information theory.