# Tag Info

$\newcommand{\ket}[1]{\left|{#1}\right\rangle}$ The $\tfrac{\theta}{2}$ can be seen on the picture given at the end of my answer. Maybe you are puzzled because you are tempting to interpret $$\Psi=\cos\frac{\theta}{2} \lvert 0\rangle + e^{i\phi}\sin\frac{\theta}{2} \lvert 1\rangle$$ as a linear combination of vectors in the 3D Euclidean space. Note ...