 Quantum depolarizing channel

A qubit depolarizing channel shrinks every Bloch vector by the same factor: ${\Lambda }_p(\rho )=p\rho +(1-p)I/2$. In this retention-parameter convention it is completely positive for $-1/3\le p\le 1$; the usual mixture of unchanged and fully randomized states uses $0\le p\le 1$.

A quantum depolarizing channel is a type of quantum channel that models a specific kind of noise affecting quantum states. It is commonly used in quantum information theory to characterize the effects of noise on quantum systems.