  A double-quantum-dot coupled to electrodes with spin-dependent splitting of
chemical potentials (spin bias) is investigated theoretically by means of the
Green's functions formalism. By applying a large spin bias, the quantum spin in
a quantum dot (the dot 1) can be manipulated in a fully electrical manner. To
noninvasively monitor the manipulation of the quantum spin in the dot 1, it is
proposed that the second quantum dot (the dot 2) is weakly coupled to the dot
1. In the presence of the exchange interaction between the two dots, the
polarized spin in the dot 1 behaves like an effective magnetic field and weakly
polarizes the spin in the nearby quantum dot 2. By applying a very small spin
bias to the dot 2, the spin-dependent transport through the dot 2 can be
probed, allowing the spin polarization in the dot 1 to be identified
nondestructively. These two steps form a complete scheme to manipulate a
trapped spin while permitting this manipulation to be monitored in the
double-dot system using pure electric approaches.
