1 $begingroup$ Pity that I wrote so a number of other points ahead of knowing what your issue was^^ You happen to be correct: Just realizing $mathcal L_A$ does not let you know how $rho_ AB $ evolves, and it could drop positivity. Be aware: Underneath is our favourite https://motivation-video93827.collectblogs.com/67634714/motivational-video-an-overview