  We design and investigate an experimental system capable of entering an
electron transport blockade regime in which a spin-triplet localized in the
path of current is forbidden from entering a spin-singlet superconductor. To
stabilize the triplet a double quantum dot is created electrostatically near a
superconducting lead in an InAs nanowire. The dots are filled stochastically
with electrons of either spin. The superconducting lead is a molecular beam
epitaxy grown Al shell. The shell is etched away over a wire segment to make
room for the double dot and the normal metal gold lead. The quantum dot closest
to the normal lead exhibits Coulomb diamonds, the dot closest to the
superconducting lead exhibits Andreev bound states and an induced gap. The
experimental observations compare favorably to a theoretical model of Andreev
blockade, named so because the triplet double dot configuration suppresses
Andreev reflections. Observed leakage currents can be accounted for by finite
temperature. We observe the predicted quadruple level degeneracy points of high
current and a periodic conductance pattern controlled by the occupation of the
normal dot. Even-odd transport asymmetry is lifted with increased temperature
and magnetic field. This blockade phenomenon can be used to study spin
structure of superconductors. It may also find utility in quantum computing
devices that utilize Andreev or Majorana states.
