Swapan Rana, Preeti Parashar, Maciej Lewenstein
Phys. Rev. A 93, 012110
http://dx.doi.org/10.1103/PhysRevA.93.012110
We show that trace distance measure of coherence is a strong monotone for all qubit and, so called, X states.
M. Friesdorf, A. H. Werner, W. Brown, V. B. Scholz, J. Eisert
arXiv:1409.1252
http://arxiv.org/abs/1409.1252