A different approach to describe energy dissipation is to consider time dependent Hamiltonians. Against a common misunderstanding, the resulting unitary dynamics can describe energy dissipation, as certain degrees of freedom loose energy and others gain energy. However, the quantum mechanical state of the system stays pure, thus such an approach can not describe dephasing unless a subsystem is chosen and the reduced density matrix of this open quantum system is analyzed. Dephasing leads to quantum decoherence or information dissipation and is often important when describing open quantum systems. However, this approach is typically used e.g. in the description of optical experiments. There a light pulse (described by a time dependent semi-classical Hamiltonian) can change the energy in the system by stimulated absorption or emission.