# Is a quantum channel essentially either a unitary evolution or a measurement?

I'd like to understand exactly what people mean when they speak of quantum channels. As I understand it, we can represent a channel by a set of Kraus operators, $$M_i$$, which satisfy $$\sum_{i}M^{\dagger}_i M_i = 1$$.

Any unitary operator (or even probabilistic combination of unitaries?) satisfies this definition. Any generalized measurement also satisfies this.

So is a quantum channel essentially either a unitary evolution or a measurement? Or is there something deeper that I am missing?

• You're not missing much. It's those two options - with a continuous slider between the two. – Emilio Pisanty Dec 14 '18 at 16:51