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arXiv:0704.0969

The Necessary and Sufficient Conditions of Separability for Bipartite Pure States in Infinite Dimensional Hilbert Spaces

Su Hu, Zong‐Wen Yu

Published Apr 7, 2007• 1 citation
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Abstract

In this paper, we present the necessary and sufficient conditions of separability for bipartite pure states in infinite dimensional Hilbert spaces. Let $M$ be the matrix of the amplitudes of $\ketψ$, we prove $M$ is a compact operator. We also prove $\ketψ$ is separable if and only if $M$ is a bounded linear operator with rank 1, that is the image of $M$ is a one dimensional Hilbert space. So we have related the separability for bipartite pure states in infinite dimensional Hilbert spaces to an important class of bounded linear operators in Functional analysis which has many interesting properties.

DOI
10.48550/arxiv.0704.0969

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In this paper, we present the necessary and sufficient conditions of separability for bipartite pure states in infinite dimensional Hilbert spaces. Let MMM be the matrix of the amplitudes of ∣ψ⟩\ketψ∣ψ⟩, we prove MMM is a compact operator. We also prove ∣ψ⟩\ketψ∣ψ⟩ is separable if and only if MMM is a bounded linear operator with rank 1, that is the image of MMM is a one dimensional Hilbert space. So we have related the separability for bipartite pure states in infinite dimensional Hilbert spaces to an important class of bounded linear operators in Functional analysis which has many interesting properties.
In this paper, the realignment criterion and the RCCN criterion of separability for states in infinite-dimensional bipartite quantum systems are established. Let HAH_AHA​ and HBH_BHB​ be complex Hilbert spaces with dim⁡HA⊗HB=+∞\dim H_A\otimes H_B=+\inftydimHA​⊗HB​=+∞. Let ρρρ be a state on HA⊗HBH_A\otimes H_BHA​⊗HB​ and {δk}\{δ_k\}{δk​} be the Schmidt coefficients of ρρρ as a vector in the Hilbert space C2(HA)⊗C2(HB){\mathcal C}_2(H_A)\otimes{\mathcal C}_2(H_B)C2​(HA​)⊗C2​(HB​). We introduce the realignment operation ρRρ^RρR and the computable cross norm ∥ρ∥CCN\|ρ\|_{\rm CCN}∥ρ∥CCN​ of ρρρ and show that, if ρρρ is separable, then ∥ρR∥Tr=∥ρ∥CCN=∑kδk≤1.\|ρ^{R}\|_{\rm Tr}=\|ρ\|_{\rm CCN}=\sum\limits_kδ_k\leq1.∥ρR∥Tr​=∥ρ∥CCN​=k∑​δk​≤1. In particular, if ρρρ is a pure state, then ρρρ is separable if and only if ∥ρR∥Tr=∥ρ∥CCN=∑kδk=1\|ρ^{R}\|_{\rm Tr}=\|ρ\|_{\rm CCN}=\sum\limits_kδ_k=1∥ρR∥Tr​=∥ρ∥CCN​=k∑​δk​=1.
Motivated by the notions of [Formula: see text]-extendability and complete extendability of the state of a finite level quantum system as described by Doherty et al. [Complete family of separability criteria, Phys. Rev. A 69 (2004) 022308], we introduce parallel definitions in the context of Gaussian states and using only properties of their covariance matrices, derive necessary and sufficient conditions for their complete extendability. It turns out that the complete extendability property is equivalent to the separability property of a bipartite Gaussian state. Following the proof of quantum de Finetti theorem as outlined in Hudson and Moody [Locally normal symmetric states and an analogue of de Finetti’s theorem, Z. Wahrscheinlichkeitstheorie und Verw. Gebiete, 33(4) (1975/76) 343–351], we show that separability is equivalent to complete extendability for a state in a bipartite Hilbert space where at least one of which is of dimension greater than 2. This, in particular, extends the result of Fannes, Lewis, and Verbeure [Symmetric states of composite systems, Lett. Math. Phys. 15(3) (1988) 255–260] to the case of an infinite dimensional Hilbert space whose C* algebra of all bounded operators is not separable.
We study the separability of bipartite quantum systems in arbitrary dimensions using the Bloch representation of their density matrix. This approach enables us to find an alternative characterization of the separability problem, from which we derive a necessary condition and sufficient conditions for separability. For a certain class of states the necessary condition and a sufficient condition turn out to be equivalent, therefore yielding a necessary and sufficient condition. The proofs of the sufficient conditions are constructive, thus providing decompositions in pure product states for the states that satisfy them. We provide examples that show the ability of these conditions to detect entanglement. In particular, the necessary condition is proved to be strong enough to detect bound entangled states.