Solution (source code)

= Solution

Conjugation by $\sigma_z$ maps $(r_x,r_y,r_z)$ to $(-r_x,-r_y,r_z)$. The <phase-flip channel> therefore acts on the <Bloch vector> as
$$
\boxed{(r_x,r_y,r_z)\longmapsto
((1-2p)r_x,(1-2p)r_y,r_z)}.
$$