Monotonicity Properties of Certain Measures over the Two-Level Quantum Systems

We demonstrate --- using the case of the two-dimensional quantum systems --- that the "natural measure on the space of density matrices describing N-dimensional quantum systems" proposed by Zyczkowski et al (quant-ph/9804024) does not belong to the class of normalized volume elements of monotone metrics on the quantum systems. Such metrics possess the statistically important property of being decreasing under stochastic mappings (coarse-grainings). We do note that the proposed natural measure (and certain evident variations upon it) exhibit quite specific monotonicity properties, but not of the form required for membership in the class.