  We propose a scheme for electrical measurement of two-electron spin states in
a semiconductor double quantum dot. We calculated the adiabatic charge transfer
when surface gates are modulated in time. Because of spin-orbit coupling in the
semiconductor, spatial displacement of the electrons causes a total spin
rotation. It follows that the expectation value of the transferred charge
reflects the relative phase as well as the total spin population of a prepared
singlet-triplet superposition state. The precise detection of the charge
transfer serves to identify the quantum superposition.
