Generally, the von Neumann entropy of a density matrix
is defined by
We use the binary logarithm (base 2) in this software.
It is also written by using the eigenvalues
This is because a density matrix can be diagonalized by a proper
unitary transformation. Such a transformation preserves eigenvalues of
. A density matrix is an Hermitian matrix and its
eigenvalues are non-negative real numbers.
shows the von Neumann entropy of the current density matrix.
shows the von Neumann entropy of the reduced density operator of the
qubits specified by bit labels.