  We present a comprehensive investigation of the polarization properties of
non-polar a-plane InGaN quantum dots (QDs) and their origin with statistically
significant experimental data and rigorous k.p modelling. The unbiased
selection and study of 180 individual QDs allow us to compute an average
polarization degree of 0.90, with a standard deviation of only 0.08. When
coupled with theoretical insights, we show that a-plane InGaN QDs are highly
insensitive to size differences, shape anisotropies, and indium content
fluctuations. Furthermore, 91% of the studied QDs exhibit a polarization axis
along the crystal [1-100] axis, with the other 9% polarized orthogonal to this
direction. When coupled with their ability to emit single-photons, a-plane QDs
are good candidates for the generation of linearly polarized single-photons, a
feature attractive for quantum cryptography protocols.
