**Purity** is a measure of how pure a quantum state is. It is often used because it is easy to compute. It is defined as *t**r*(*ρ*^{2}).

The purity is one for pure states and 1/*d* for maximally mixed states of dimension *d*.

Related to the purity is the linear entropy, which measures the mixedness of a state.

## Last modified:

Monday, October 26, 2015 - 17:56