pub fn density_quantum_fluctuation(
operator: &(impl LinearOperator + ?Sized),
density: &[Complex64],
) -> Result<Complex64>Expand description
Density-matrix fluctuation Tr(ρ A²) - Tr(ρ A)².
The density matrix follows the same row-major convention as
density_expectation. Normalization is deliberately not imposed: callers
may use normalized density matrices or preserve QuSpin’s direct linear
algebra semantics for weighted ensembles.