# Quantum linear response theory


Consider again the Hamiltonian for a system coupled to a time-dependent field

$H = H_0 - BF_e(t)$

We wish to solve the quantum Liouville equation

$i\hbar {\partial \rho \over \partial t} = [H,\rho]$

in the linear regime where $$F_e(t)$$ is small.