Part IX — Noise: The Real Computer
Everything so far assumed unitary evolution of a perfectly isolated state vector. Real devices are open systems in constant contact with an environment they cannot control. This part builds the working language of noise: the failure mechanisms themselves (29), the state object that can actually represent them (30), and the operators that propagate them step by step (31). Density matrices and quantum channels are the objects you will simulate when you want to be honest about a real machine.