Let's consider a two state quantum system in which an electron is either in the ground state or in the excited state with the energy difference E. We know that if we put this system in a monochromatic electromagnetic field (with frequency E/h) the electron can become excited by means of stimulated absorption of a photon. We know that if the electron is in the excited state it can "fall" to the ground state by emitting a photon either by means of the stimulated emission in which a photon of the electromagnetic field stimulates the electron and two photons gets emitted or by spontaneous emission (which in turn is stimulated too, because it is stimulated by the vacuum field fluctuations of QED). My question is the following. Since we know that spontaneous emission is not spontaneous at all but has its origins in the stimulation due to the vacuum field fluctuations, what hinders spontaneous absorption of a photon of this fluctuating field? I don't really see why this in fact wouldn't be possible. I do know that spontaneous absorption is not something that one talks about when for instance calculating Einstein's A and B coefficients, neither when calculating Rabi oscillations. This makes me thing that if spontaneous absorption is possible, it should be far less probable. If this is the case, what makes it so improbable?