I'm working on a teleport protocol and I need to open the matrix of each operator, however, there's a CZ gate between q0 and q2 at the end of it and I don't know how to write the matrix for it and operate in the state.
This is the protocol, really basic, and the CZ is the last one.
Consider that a control qubit is $q_k$ and a target qubit is $q_{k+n}$ and you want to apply operator $U$ on the target qubit. Denote $N=2^{n+1}$. Then matrix representation of this controlled $U$ is \begin{equation} CU= \begin{pmatrix} I_{\frac{N}{2}} & O_{\frac{N}{2}} \\ O_{\frac{N}{2}} & I_{\frac{N}{4}} \otimes U \\ \end{pmatrix} \end{equation}
In your case $U=Z=\begin{pmatrix}1 & 0 \\ 0 & -1\end{pmatrix}$, $k=0$ and $n=2$, so the matrix representation of operator $Z$ acting on $q_{2}$ controlled by $q_{0}$ is
\begin{equation} \begin{pmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & -1 & 0 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & 0 & 0 & 0 & 0 & -1 \\ \end{pmatrix} \end{equation}
You can write the controlled-phase gate, as applied to just those two qubits, as $$ I\otimes I-2|1\rangle\langle 1|\otimes |1\rangle\langle 1|. $$ If you want these to act on qubits 1 and 3, then you need to apply identity, $I$, on the second qubit: $$ I\otimes I\otimes I-2|1\rangle\langle 1|\otimes I\otimes |1\rangle\langle 1|. $$
What you need is:
In terms of matrices, all you need is to multiply 3 8x8 matrices.
Hope it will help.
