  Asymmetry in a three-electron double quantum dot (DQD) allows spin blockade,
when spin-3/2 (quadruplet) states and spin-1/2 (doublet) states have different
charge configurations. We have observed this DQD spin blockade near the
(1,2)-(2,1) charge transition using a pulsed-gate technique and a charge
sensor. We then use this spin blockade to detect Landau-Zener-St\"uckelberg
(LZS) interference and coherent oscillations between the spin quadruplet and
doublet states. Such studies add to our understandings of coherence and control
properties of three-spin states in a double dot, which in turn would benefit
the explorations into various qubit encoding schemes in semiconductor
nanostructures.
