Solution
= Solution
The <reduced density matrix> has two <eigenvalues> equal to $1/2$. Its <Von Neumann entropy>, which is the <entanglement entropy> of the bipartite <pure state>, is
$$
\boxed{S_R=-\operatorname{Tr}(\rho_R\ln\rho_R)=-2\left(\frac12\ln\frac12\right)=\ln2.}
$$
This is one bit when base-two logarithms are used. It is the maximum <entanglement entropy> for a pair of two-dimensional systems, while the joint <Von Neumann entropy> of the <singlet state> remains zero.