I am completely new to quantum computing and playing QuantAttack. While playing, I found something strange: SZS turns into Z.
According to the Wikipedia article that the game recommends reading, SZS must disappear because:
$$ SZS = \begin{pmatrix}1 & 0 \\ 0 & i\end{pmatrix} \begin{pmatrix}1 & 0 \\ 0 & -1\end{pmatrix} \begin{pmatrix}1 & 0 \\ 0 & i\end{pmatrix} = \begin{pmatrix}1 & 0 \\ 0 & 1\end{pmatrix} = I $$
Is there some weird quantum thing I missed, or is it just a game bug?
SZS is not equal to Z, it is equal to identity matrix as mentioned in the article. SZ$S^{\dagger}$ is equal to Z ,where $S^{\dagger}$ is the conjugate transpose of S gate.
This can be proved by matrix multiplication $$ SZS^{\dagger}=\begin{pmatrix}1 & 0 \\ 0 & i\end{pmatrix} \begin{pmatrix}1 & 0 \\ 0 & -1\end{pmatrix} \begin{pmatrix}1 & 0 \\ 0 & -i\end{pmatrix} = \begin{pmatrix}1 & 0 \\ 0 & -1\end{pmatrix} = Z $$
You can check the below link for more info about S and S dagger gates https://www.quantum-inspire.com/kbase/s-gate/
