Complex Hadamard matrix

Complex Hadamard matrices are any complex $N\times N$ matrix $H$ satisfying two conditions:

• unimodularity: $|H_{jk}|=1{\quad {\rm {for\quad }}}j,k=1,2,\dots ,N$

• orthogonality: $HH^{\dagger }=N\;{\mathbb {I} }$,

where ${\dagger }$ denotes the Hermitian transpose of H and ${\mathbb {I} }$ is the identity matrix. Complex Hadamard matrices arise in the study of operator algebras and the theory of quantum computation. Real Hadamard matrices and Butson-type Hadamard matrices form particular cases of complex Hadamard matrices.

Complex Hadamard matrices exists for any natural N. For instance the Fourier matrices,

$[F_{N}]_{jk}:=\exp[(2\pi i(j-1)(k-1)/N]{\quad {\rm {for\quad }}}j,k=1,2,\dots ,N$

belong to this class.

Equivalent

Two complex Hadamard matrices are called equivalent, written $H_{1}\simeq H_{2}$, if there exist diagonal unitary matrices $D_{1},D_{2}$ and permutation matrices $P_{1},P_{2}$ such that

$H_{1}=D_{1}P_{1}H_{2}P_{2}D_{2}$.

Any complex Hadamard matrix is equivalent to a dephased Hadamard matrix, in which all elements in the first row and first column are equal to unity.

For $N=2,3$ and $5$ all complex Hadamard matrices are equivalent to the Fourier matrix $F_{N}$. For $N=4$ there exists a continuous, one-parameter family of inequivalent complex Hadamard matrices,

$F_{4}^{(1)}(a):={\begin{bmatrix}1&1&1&1\\1&ie^{ia}&-1&-ie^{ia}\\1&-1&1&-1\\1&-ie^{ia}&-1&ie^{ia}\end{bmatrix}}{\quad {\rm {with\quad }}}a\in [0,\pi ).$

For $N=6$ the following not equivalent complex Hadamard matrices are known:

• a single two-parameter family which includes $F_{6}$,
• a single one-parameter family
• a one-parameter orbit including the circulant Hadamard matrix $C_{6}$,
• a single point - one of the Butson-type Hadamard matrices, $S_{6}\in H(3,6)$.

It is not known, however, if this list is complete.

References

• U. Haagerup, Orthogonal maximal abelian *-subalgebras of the n×n matrices and cyclic n-roots, Operator Algebras and Quantum Field Theory (Rome), 1996 (Cambridge, MA: International Press) pp 296-322.
• P. Dita, Some results on the parametrization of complex Hadamard matrices, J. Phys. A: Math. Gen. 37, 5355-5374 (2004).

External Links

• For an explicit list of known $N=6$ complex Hadamard matrices and several examples of Hadamard matrices of size 7-16 see Catalogue of Complex Hadamard Matrices