Channel

Initial state

Bit flip:
rho -> (1-p) rho + p X rho X
Bloch: x unchanged, y,z contracted

Phase flip:
rho -> (1-p) rho + p Z rho Z
Bloch: z unchanged, x,y contracted

Depolarizing:
rho -> (1-p) rho + p I/2
Bloch sphere uniformly shrinks

Amplitude damping:
|1> -> |0> with rate gamma
Bloch: asymmetric decay toward north pole