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Mar
7
comment mixture of maximally mixed and maximally entangled state
This answer is not the correct one (this besides the fact that it contains a mathematical mistake). One has to prove that when the state is PPT, it is also separable. This is possible by checking that in the range where it is PPT, the state has a separable expression. This can be done by considering the action of a UU* twirling (an LOCC operation) on an appropriately chosen factorized pure state.
Feb
14
comment What Hermitian operators can be observables?
You can impose other superselection rules, typically based on some symmetry, not necessarily the exchange symmetry (that applies only for multi-particle systems). I cannot recommend a reference based on my direct knowledge, but I'd say you might start from rmp.aps.org/abstract/RMP/v79/i2/p555_1
Feb
14
comment Can the concurrence be calculated in terms of the entanglement of formation?
By "does not look as nice as" I actually meant that you cannot write it down as a combination of elementary functions. Even just inverting the binary entropy $-x\log x - (1-x)\log (1-x)$ is not possible in this sense, as far as I know. You can just show that the inverse exists, and maybe calculate some properties, or try to expand it in series, but I believe that's it.
Jan
26
comment Do any entanglement measures for mixed states exist that use only single site correlation functions?
As Joe says, the local spectrum will still tell you something about entanglement if you have some kind of promise/information about the global purity. For example, if $S(\rho_{AB})<S(\rho_A)$, then the state is $A:B$-entangled
Sep
29
comment Does entropy measure extractable work?
More recent results here arxiv.org/abs/0908.0424 and here arxiv.org/abs/1009.1630