Pulsed-gate measurements of the singlet-triplet relaxation time in a two-electron double quantum dot

We use a pulsed-gate technique to measure the singlet-triplet relaxation time in a two-electron double quantum dot when the singlet and triplet states are nearly degenerate. Transitions from the (1,1) to (0,2) charge state involve spin selection rules. Measurements of this transition probability as a function of pulse time and perpendicular magnetic field are used to determine the (1,1) singlet-triplet relaxation time and the (0,2) singlet-triplet splitting. We find a singlet-triplet relaxation time $\geq$70 $\mu$s for our double dot. Experiments aimed at measuring the spin T$_2$ time will be described.