Volumes of conditioned bipartite state spaces
  We analyse the metric properties of $\textit{conditioned}$ quantum state spaces $\mathcal{M}^{(n\times m)}_{\eta}$. These spaces are the convex sets of $nm \times nm$ density matrices that, when partially traced over $m$ degrees of freedom, respectively yield the given $n\times n$ density matrix $\eta$. For the case $n=2$, the volume of $\mathcal{M}^{(2\times m)}_{\eta}$ equipped with the Hilbert-Schmidt measure is a simple polynomial of the radius of $\eta$ in the Bloch-Ball. Remarkably, the probability $p_{\mathrm{sep}}^{(2\times m)}(\eta)$ to find a separable state in $\mathcal{M}^{(2\times m)}_{\eta}$ is independent of $\eta$ (except for $\eta$ pure). Both these results are proven analytically for the case of the family of $4\times 4$ $X$-states, and thoroughly numerically investigated for the general case. The important implications of these results for the clarification of open problems in quantum theory are pointed out and discussed. 
Simon Milz and Walter T. Strunz
quant-ph