Solution
= Solution
The <Pauli Z gate> gives a \b[rotation through $\pi$ about the $z$ axis] of the <Bloch sphere>. In <Bloch vector> coordinates,
$$
\boxed{(r_x,r_y,r_z)\longmapsto(-r_x,-r_y,r_z).}
$$
For example, this follows from $ZXZ=-X$, $ZYZ=-Y$ and $ZZZ=Z$ in the <Pauli matrix> expansion $\rho=(I+r_xX+r_yY+r_zZ)/2$.