The four <Bell states> form an orthonormal basis, so their uniform mixture is
$$
\rho=\frac{I_4}{4}
=\frac{I_2}{2}\otimes\frac{I_2}{2}.
$$
It is a product state and therefore
$$
\boxed{\operatorname{SN}(\rho)=1}.
$$

Solution

Past exam of the mathematics course of the University of Cambridge / 2021 / iii / Paper 323 / 1 / b / i / 3 / Solution

 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2021/iii/paper-323/1/b/i/3/solution