In quantum statistical mechanics, the concept of entropy was developed by John von Neumann and is generally referred to as " von Neumann entropy ": S = − − k B t r ( ρ ρ ^ ^ × × ln ⁡ ⁡ ρ ρ ^ ^ ) where ρ ρ ^ ^ is the density matrix, t r is the trace operator and k B is the Boltzmann constant.