A Complete Classification of Complex Hadamard Matrices of Order Six

arXiv:2608.18053v1 Announce Type: new Abstract: Complex Hadamard matrices encode perfectly balanced unitary transformations. They underlie mutually unbiased quantum measurements and multiphoton interferometry. Their classification is complete through order five, but order six -- the first dimension in which several continuous families coexist with an isolated solution -- has remained open for decades. Here, we give a complete and exact finite-incidence classification of order-six complex Hadamard matrices up to standard equivalence. We first prove that every such matrix can be constructed from an initial dephased $3 \times 3$ corner by a finite, branch-complete procedure. This supplies the global step missing from Sz\"oll\H{o}si's dilation method and proves his conjecture: up to standard equivalence, every class outside Karlsson's three-parameter family and Tao's isolated matrix is recovered algebraically from a suitable corner. We then describe the geometry of the reconstruction from four initial phases and show that, except for Tao's isolated matrix and a single explicit Karlsson matrix, every class admits a representative obtained by solving one quadratic and one cubic equation in both the horizontal and vertical directions. Our work resolves the classification problem and provides a rigorous framework for further investigating order-six Hadamards, with applications to balanced six-mode interferometers and the study of mutually unbiased bases.

Abstract:Complex Hadamard matrices encode perfectly balanced unitary transformations. They underlie mutually unbiased quantum measurements and multiphoton interferometry. Their classification is complete through order five, but order six -- the first dimension in which several continuous families coexist with an isolated solution -- has remained open for decades. Here, we give a complete and exact finite-incidence classification of order-six complex Hadamard matrices up to standard equivalence. We first prove that every such matrix can be constructed from an initial dephased $3 \times 3$ corner by a finite, branch-complete procedure. This supplies the global step missing from Szöllősi's dilation method and proves his conjecture: up to standard equivalence, every class outside Karlsson's three-parameter family and Tao's isolated matrix is recovered algebraically from a suitable corner. We then describe the geometry of the reconstruction from four initial phases and show that, except for Tao's isolated matrix and a single explicit Karlsson matrix, every class admits a representative obtained by solving one quadratic and one cubic equation in both the horizontal and vertical directions. Our work resolves the classification problem and provides a rigorous framework for further investigating order-six Hadamards, with applications to balanced six-mode interferometers and the study of mutually unbiased bases.

Quantum Physics (quant-ph); Mathematical Physics (math-ph); Combinatorics (math.CO)

arXiv:2608.18053 [quant-ph]

(or arXiv:2608.18053v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2608.18053 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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A Complete Classification of Complex Hadamard Matrices of Order Six

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