Existence of symmetric informationally complete (SIC) measurements in all finite dimensions

Establish whether symmetric informationally complete positive operator-valued measures (SIC-POVMs) exist in every finite Hilbert-space dimension N; specifically, determine for arbitrary N≥2 the existence of a set of N^2 rank‑1 projectors {Πj} satisfying Tr(Πi Πj) = (δi,j N + 1)/(N + 1), thereby resolving the general existence question for SIC measurements.

Background

The text introduces informationally complete (IC) measurements and highlights a special class—symmetric informationally complete (SIC) measurements—characterized by a specific overlap condition, Tr ΠiΠj = (δi,j N + 1)/(N + 1). These POVMs are central to quantum tomography and cryptography because they enable unique reconstruction of quantum states from measurement statistics.

While SIC-POVMs are known to exist in many specific dimensions, the general existence problem across all finite dimensions remains unresolved. The paper explicitly notes this uncertainty, framing it as an open question tied to fundamental aspects of quantum measurement theory and practical applications in quantum information processing.

References

We still do not know if SIC exists in any dimension.

— Quantum machine learning -- lecture notes  (2512.05151 - Žunkovič, 3 Dec 2025) in Section: General measurements/events, Example (informationally complete measurement), footnote

Exact SICs are known in many dimensions, but unconditional existence in every dimension remains open.

— Uniformly Stable Minimal Weyl--Heisenberg Measurements Approaching the SIC Benchmark  (2608.11850 - Zhu et al., 12 Aug 2026) in Section 1, Introduction

Zauner conjectured that such a system exists in every dimension, cf.. So far, only a finite number of maximal ETFs are known, and the conjecture remains open.

— Required Number of Points in $L_2$ Marcinkiewicz-Zygmund Inequalities  (2608.25886 - Bartel, 26 Aug 2026) in Section 3.3, “Maximal complex ETFs”

Another construction for 2-designs is via the generalisation of the SIC POVM. The latter consist of d² states---the minimal cardinality for a 2-design---and are conjectured to exist for any dimension [...], but are only constructed explicitly for dimensions up to about d=67 [...].

— Quantifying the fidelity of a quantum memory  (2610.08445 - Barizien et al., 6 Oct 2026) in Section 4.2, “Witnesses for higher dimensions”