Function RZZGate 

pub fn RZZGate() -> UnitaryExpression

The two-qubit RZZ gate (Ising ZZ coupling).

$$ RZZ(\theta) = e^{-i\theta/2 , Z \otimes Z} = \begin{pmatrix} e^{-i\theta/2} & 0 & 0 & 0 \\ 0 & e^{i\theta/2} & 0 & 0 \\ 0 & 0 & e^{i\theta/2} & 0 \\ 0 & 0 & 0 & e^{-i\theta/2} \end{pmatrix} $$