  Quantum dots are arguably one of the best platforms for optically accessible
spin based qubits. The paramount demand of extended qubit storage time can be
met by using quantum-dot-confined dark exciton: a longlived electron-hole pair
with parallel spins. Despite its name the dark exciton reveals weak
luminescence that can be directly measured. The origins of this optical
activity remain largely unexplored. In this work, using the atomistic
tight-binding method combined with configuration-interaction approach, we
demonstrate that atomic-scale randomness strongly affects oscillator strength
of dark excitons confined in self-assembled cylindrical InGaAs quantum dots
with no need for faceting or shape-elongation. We show that this process is
mediated by two mechanisms: mixing dark and bright configurations by exchange
interaction, and equally important appearance of non-vanishing optical
transition matrix elements that otherwise correspond to nominally forbidden
transitions in a non-alloyed case. The alloy randomness has essential impact on
both bright and dark exciton states, including their energy, emission
intensity, and polarization angle. We conclude that, due to the atomic-scale
alloy randomness, finding dots with desired dark exciton properties may require
exploration of a large ensemble, similarly to how dots with low bright exciton
splitting are selected for entanglement generation.
