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I am following a tutorial in Given rotations. Suppose I have the quantum state $|100\rangle$ and would like to use two Givens $G$ with angle $\theta, \phi$ rotation to transform it into a superposition of $\alpha|100\rangle+\beta|010\rangle+\gamma|001\rangle$. I denote $\cos(\frac\cdot 2), \sin(\frac\cdot 2)$ as $c(\cdot), s(\cdot)$

Consider $G(1,4,\phi)$ which maps $|100\rangle$ to $|001\rangle$ $$G(1,4,\phi)=\begin{bmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & c(\phi) & 0 & 0 & -s(\phi)& 0 & 0 & 0\\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & 0 & 1 & 0 & 0& 0 & 0\\ 0 & s(\phi)& 0 & 0 & c(\phi) & 0& 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 1& 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 0& 1 & 0\\ 0 & 0 & 0 & 0 & 0 & 0& 0 & 1 \end{bmatrix}$$ and $G(2,4,\theta)$ which maps $|100\rangle$ to $|010\rangle$ $$G(2,4,\theta)=\begin{bmatrix} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0\\ 0 & 0 & c(\theta)& 0 & -s(\theta)& 0 & 0 & 0\\ 0 & 0 & 0 & 1 & 0 & 0& 0 & 0\\ 0 & 0 & s(\theta)& 0 & c(\theta)& 0& 0 &0\\ 0 & 0 & 0 & 0 & 0 & 1& 0 & 0\\ 0 & 0 & 0 & 0 & 0 & 0& 1 & 0\\ 0 & 0 & 0 & 0 & 0 & 0& 0 & 1 \end{bmatrix}$$

$G(2,4,\phi)G(1,2,\theta)$ is the mapping that I needs, $$G:=G(2,4,\phi)G(1,4,\theta)=\begin{bmatrix} 1&0&0&0&0&0&0&0\\ 0&c(\phi)&0&0&-s(\phi)&0&0&0\\ 0&-s(θ)s(\phi)&c(θ)&0&-cθs(\phi)&0&0&0\\ 0&0&0&1&0&0&0&0\\ 0&c(θ)s(\phi)&s(θ)&0&c(θ)c(\phi)&0&0&0\\ 0&0&0&0&0&1&0&0\\ 0&0&0&0&0&0&1&0\\ 0&0&0&0&0&0&0&1\end{bmatrix}$$

Now $G|100\rangle=-s(\phi)|001\rangle-s(\phi) c(\theta)|010\rangle +c(\theta) c(\phi) |100\rangle$. However the supposed solution is $c(\theta)s(\phi)|001\rangle - s(\theta)|010\rangle + c(\theta)c(\phi)|100\rangle$. I don't understand how the 1st and 2nd coefficient is that way

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  • $\begingroup$ First you set up $G(1,4,\phi)$ and $G(2,4,\theta)$, but then you use $G(2,4,\phi)$ and $G(1,2,\theta)$ – is that on purpose? Anyway, I suspect you'll get the desired answer if you swap the factors in the product. $\endgroup$
    – joriki
    Commented Feb 7 at 19:51

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