We propose to quantify the entanglement of pure states of $N\times N$ bipartite quantum systems by defining its Husimi distribution with respect to $\mathrm{SU}\mathrm{}\left(N\right)\mathrm{}\times \mathrm{SU}\mathrm{}\left(N\right)\mathrm{}$ coherent states. The Wehrl entropy is minimal if and only if the analyzed pure state is separable. The excess of the Wehrl entropy is shown to be equal to the subentropy of the mixed state obtained by partial trace of the bipartite pure state. This quantity, as well as the generalized (Rényi) subentropies, are proved to be Schur concave, so they are entanglement monotones and may be used as alternative measures of entanglement.

• Received 2 September 2003

DOI:https://doi.org/10.1103/PhysRevA.69.022317