Bures Measures over the Spaces of Two and Three-Dimensional Density Matrices

Due to considerable recent interest in the use of density matrices for a wide variety of purposes, including quantum computation, we present a general method for their parameterizations in terms of Euler angles. We assert that this is of more fundamental importance than (as several people have remarked to us) ``just another parameterization of the density matrix.'' There are several uses to which this methodology can be put. One that has received particular attention is in the construction of certain distinguished (Bures) measures on the $(n^2 -1)$-dimensional convex sets of $n \times n$ density matrices.