Simplified exact SICs

Abstract

In the standard basis exact expressions for the components of SIC vectors (belonging to a symmetric informationally complete POVM) are typically very complicated. We show that a simple transformation to a basis adapted to the symmetries of a fiducial SIC vector can result in a massive reduction in complexity. We rely on a conjectural number theoretic connection between SICs in dimension $d_j$ and SICs in dimension $d_{j+1} = d_j(d_j-2)$. We focus on the sequence 5, 15, 195, ... . We rewrite Zauner's exact solution for the SIC in dimension 5 to make its simplicity manifest, and use our adapted basis to convert numerical solutions in dimensions 15 and 195 to exact solutions. Comparing to the known exact solutions in dimension 15 we find that the simplification achieved is dramatic. The proof that the exact vectors are indeed SIC fiducial vectors, also in dimension 195, is a long calculation guided by the standard ray class hypothesis about the algebraic number fields generated by the SICs. We conjecture that our result generalizes to every dimension in the particular sequence we consider.