1, 2, and 6 qubits, and the Ramanujan-Nagell theorem

Abstract

A conjecture of Ramanujan that was later proved by Nagell is used to show on the basis of matching dimensions that only three $n$-qubit systems, for $n=1,2,6$, can share an isomorphism of their symmetry groups with the rotation group of corresponding dimensions $3,6,91$. Topological analysis, however, rules out the last possibility.