Function RXXGate 

pub fn RXXGate() -> UnitaryExpression

The two-qubit RXX gate (Ising XX coupling).

$$ RXX(\theta) = e^{-i\theta/2 , X \otimes X} = \begin{pmatrix} \cos(\theta/2) & 0 & 0 & -i\sin(\theta/2) \\ 0 & \cos(\theta/2) & -i\sin(\theta/2) & 0 \\ 0 & -i\sin(\theta/2) & \cos(\theta/2) & 0 \\ -i\sin(\theta/2) & 0 & 0 & \cos(\theta/2) \end{pmatrix} $$