The von Neumann entropy of a quantum state ρ is given by the formula,
S(ρ) ≡ − Tr(ρlogρ)
and if λi are the eigenvalues of ρ then the Von Neumann entropy can be reexpressed as:
S(ρ) = − ∑iλilog(λi).
In quantum information the logarithms are usually taken to be base 2, giving a maximum entropy of 1 for a qubit.