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Towards a classification of $6\times 6$ complex Hadamard matrices

Máté Matolcsi·Ferenc Szöllősi
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Abstract

Complex Hadamard matrices have received considerable attention in the past few years due to their appearance in quantum information theory. While a complete characterization is currently available only up to order 5 (in \cite{haagerup}), several new constructions of higher order matrices have appeared recently \cite{dita, karol, BN, MM, sz}. In particular, the classification of {\it self-adjoint} complex Hadamard matrices of order 6 was completed by Beuachamp and Nicoara in \cite{BN}, providing a previously unknown non-affine one-parameter orbit. In this paper we classify all \emph{dephased, symmetric} complex Hadamard matrices \emph{with real diagonal} of order 6. Furthermore, relaxing the condition on the diagonal entries we obtain a new non-affine one-parameter orbit connecting the Fourier matrix $F_6$ and Diţă's matrix $D_6$. This answers a recent question of Bengtsson & al. in \cite{BB}.

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Máté Matolcsi, Ferenc Szöllősi. 2007-02-02. Towards a classification of $6\times 6$ complex Hadamard matrices. https://arxiv.org/abs/math/0702043

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