Some families of density matrices for which separability is easily tested

Samuel L. Braunstein, Sibasish Ghosh, Toufik Mansour, Simone Severini, Richard C. Wilson

Research output: Contribution to journalArticlepeer-review


We reconsider density matrices of graphs as defined in quant-ph/0406165. The density matrix of a graph is the combinatorial Laplacian of the graph normalized to have unit trace. We describe a simple combinatorial condition (the "degree condition") to test the separability of density matrices of graphs. The condition is directly related to the Peres-Horodecki partial transposition condition. We prove that the degree condition is necessary for separability, and we conjecture that it is also sufficient. We prove special cases of the conjecture involving nearest-point graphs and perfect matchings. We observe that the degree condition appears to have a value beyond the density matrices of graphs. In fact, we point out that circulant density matrices and other matrices constructed from groups always satisfy the condition and indeed are separable with respect to any split. We isolate a number of problems and delineate further generalizations.

Original languageEnglish
Article number012320
JournalPhysical Review A - Atomic, Molecular, and Optical Physics
Issue number1
StatePublished - 2006

ASJC Scopus subject areas

  • Atomic and Molecular Physics, and Optics


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