Geometric count conjecture for Weyl–Heisenberg SICs

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Fix d∈Z>0d\in\mathbb Z_{>0}, d≠3d\neq3, let Δ=(d+1)(d−3)\Delta=(d+1)(d-3), and let OΔ\mathcal O_\Delta be the quadratic order of discriminant Δ\Delta. Geometric Count Conjecture. The number of geometric equivalence classes of Weyl–Heisenberg covariant SICs is ∣WHSIC⁡(d)/PEC(d)∣=∣ClM⁡(OΔ)∣|\operatorname{WHSIC}(d)/\mathcal{PEC}(d)|=|\operatorname{ClM}(\mathcal O_\Delta)|. This connects the enumeration of SICs up to geometric equivalence with the class monoid of a quadratic order; it remains open in general.

References

Primary source

Gene S. Kopp and Jeffrey C. Lagarias, “SIC-POVMs and orders of real quadratic fields”, arXiv:2407.08048 (2026).

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