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