Jacques Hadamard

Jacques Hadamard was a great French Mathematician. He was a great-uncle of Laurent Schwartz, who reports numerous fantastic anecdotes about him in his biography. This one, of Hadamard looking for a job as a refugee from Nazi Europe, is my that is, Fabrice's. favourite:

He is famous in Quantum Information Processing for the Hadamard Gate $$\begin{equation}

 H=
 {1\over{\sqrt{2}}}
 \begin{pmatrix}
   1 & 1\\
   1 & -1
 \end{pmatrix}\,.

\end{equation} $$ which is named after him as a particular case of his Hadamard matrices (matrices of $\pm1$ entries with perpendicular vectors).