Characteristics and benchmarks of entanglement of mixed states -- the   two qubit case
  We propose that the entanglement of mixed states is characterised properly in terms of a probability density function $\mathcal{P}_{\rho}(\mathcal{E})$. There is a need for such a measure since the prevalent measures (such as \textit{concurrence} and \textit{negativity}) for two qubit systems are rough benchmarks, and not monotones of each other. Focussing on the two qubit states, we provide an explicit construction of $\mathcal{P}_{\rho}(\mathcal{E})$ and show that it is characterised by a set of parameters, of which concurrence is but one particular combination. $\mathcal{P}_{\rho}(\mathcal{E})$ is manifestly invariant under $SU(2) \times SU(2)$ transformations. It can, in fact, reconstruct the state up to local operations - with the specification of at most four additional parameters. Finally the new measure resolves the controversy regarding the role of entanglement in quantum computation in NMR systems. 
Shanthanu Bhardwaj and V. Ravishankar