  We consider a quantum dot attached to leads in the Coulomb blockade regime
which has a spin 1/2 ground state. We show that by applying an ESR field to the
dot-spin the stationary current in the sequential tunneling regime exhibits a
resonance whose line width is determined by the single-spin decoherence time
T_2. The Rabi oscillations of the dot-spin are shown to induce coherent current
oscillations from which T_2 can be deduced in the time domain. We describe a
spin-inverter which can be used to pump current through a double-dot via spin
flips generated by ESR.
