On small proofs of Bell-Kochen-Specker theorem for two, three and four qubits
Résumé
Bell-Kochen-Specker theorem (BKS) theorem rules out realistic {\it non-contextual} theories by resorting to impossible assignments of rays among a selected set of maximal orthogonal bases. We investigate the geometrical structure of small BKS-proofs $v-l$ involving $v$ real rays and $l$ $2n$-dimensional bases of $n$-qubits ($1< n < 5$). Specifically, we look at the parity proof $18-9$ with two qubits (A. Cabello, 1996 \cite{Cabello1996}), the parity proof $36-11$ with three qubits (M. Kernaghan \& A. Peres, 1995 \cite{Kernaghan1965}) and a newly discovered non-parity proof 80-21 with four qubits (that improves a work at P.~K Aravind's group in 2008 \cite{Harvey2008}). The rays in question arise as real eigenstates shared by some maximal commuting sets (bases) of operators in the $n$-qubit Pauli group. One finds universal signatures of the distances among the bases, that carry various symmetries in their graphs.
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