The **von Neumann entropy** of a quantum state *ρ* is given by the formula,

S(*ρ*) ≡ − Tr(*ρ*log*ρ*)

and if *λ*_{i} are the eigenvalues of *ρ* then the Von Neumann entropy can be reexpressed as:

S(*ρ*) = − ∑_{i}*λ*_{i}log(*λ*_{i}).

In quantum information the logarithms are usually taken to be base 2, giving a maximum entropy of 1 for a qubit.

### See also

## Last modified:

Monday, October 26, 2015 - 17:56