  We detect a novel radiative cascade from a neutral semiconductor quantum dot.
The cascade initiates from a metastable biexciton state in which the holes form
a spin-triplet configuration, Pauli-blockaded from relaxation to the
spin-singlet ground state. The triplet biexciton has two photon-phonon-photon
decay paths. Unlike in the singlet-ground state biexciton radiative cascade, in
which the two photons are co-linearly polarized, in the triplet biexciton
cascade they are crosslinearly polarized. We measured the two-photon
polarization density matrix and show that the phonon emitted when the
intermediate exciton relaxes from excited to ground state, preserves the
exciton's spin. The phonon, thus, does not carry with it any which-path
information other than its energy. Nevertheless, entanglement distillation by
spectral filtering was found to be rather ineffective for this cascade. This
deficiency results from the opposite sign of the anisotropic electron-hole
exchange interaction in the excited exciton relative to that in the ground
exciton.
