  A geometric characterization is given for invertible quantum measurement
maps. Denote by ${\mathcal S}(H)$ the convex set of all states (i.e., trace-1
positive operators) on Hilbert space $H$ with dim$H\leq \infty$, and $[\rho_1,
\rho_2]$ the line segment joining two elements $\rho_1, \rho_2$ in ${\mathcal
S}(H)$. It is shown that a bijective map $\phi:{\mathcal S}(H) \rightarrow
{\mathcal S}(H)$ satisfies $\phi([\rho_1, \rho_2]) \subseteq
[\phi(\rho_1),\phi(\rho_2)]$ for any $\rho_1, \rho_2 \in {\mathcal S}$ if and
only if $\phi$ has one of the following forms $$\rho \mapsto \frac{M\rho
M^*}{{\rm tr}(M\rho M^*)}\quad \hbox{or} \quad \rho \mapsto \frac{M\rho^T
M^*}{{\rm tr}(M\rho^T M^*)},$$ where $M$ is an invertible bounded linear
operator and $\rho^T$ is the transpose of $\rho$ with respect to an arbitrarily
fixed orthonormal basis.
