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arXiv:quant-ph/0210115

Constraints on the mixing of states on bipartite quantum systems

Chen, Hao

Published Oct 15, 2002• 0 citations
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Abstract

We give necessary conditions for the mixing problem in bipartite case, which are independent of eigenvalues and based on algebraic-geometric invariants of the bipartite states. One implication of our results is that for some special bipartite mixed states, only special mixed states in a measure zero set can be used to mix to get them. The results indicate for many physical problems on composite quantum systems the description based on majorization of eigenvalues is not sufficient

DOI
10.48550/arxiv.quant-ph/0210115

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We give necessary conditions for the mixing problem in bipartite case, which are independent of eigenvalues and based on algebraic-geometric invariants of the bipartite states. One implication of our results is that for some special bipartite mixed states, only special mixed states in a measure zero set can be used to mix to get them. The results indicate for many physical problems on composite quantum systems the description based on majorization of eigenvalues is not sufficient
A mixed state may be represented in many different ways as a mixture of pure states ρ=∑pi∣ψi⟩⟨ψi∣. The mixing problem in quantum mechanics asks the characterization of the probability distribution (pi) and the mixed states (ρi) such that ρ=∑piρi for any given mixed state ρ. Some constraints based on eigenvalues of the mixed states are established in uni-party case [see Nielsen, Phys. Rev. A. 63, 052308 (2000), 63, 022144 (2000), Nielsern and Vidal Quantum Inf. Comput. 1 76 (2001)]. We develop some new invariant sets for bipartite mixed states under local unitary operations, which are independent of eigenvalues, and prove some strong constraints based on these invariant sets for the mixing problem in bipartite case. This exhibits a remarkable difference from the uni-party case.
From the consideration of measuring bipartite mixed states by separable pure states, we introduce algebraic sets in complex projective spaces for bipartite mixed states as the degenerating locus of the measurement. These algebraic sets are independent of the eigenvalues and only measure the "position" of eigenvectors of bipartite mixed states. They are nonlocal invariants ie., remaining invariant after local unitary transformations. The algebraic sets have to be the sum of the linear subspaces if the mixed states are separable, and thus we give a "eigenvalue-free" criterion of separability.Based on our criterion, examples are given to illustrate that entangled mixed states which are invariant under partial transposition or fufill entropy and disorder criterion of separability can be constructed systematically.We reveal that a large part of quantum entanglement is independent of eigenvalue spectra and develop a method to measure this part of quantum enatnglement.The results are extended to multipartite case.