# Photons from resonant biexciton excitation as basis for entanglement

I'm currently reading this paper on generating time-bin entanglement from the photons emitted by a quantum dot (QD). The process described uses a resonant excitation of a biexciton state of said quantum dot.

The resonant laser pump pulse can transfer the phase of the pump pulses (that are generated by a interferometer before exciting the QD) and thus the coherence created by the pump interferometer to the biexciton state.

The problem I have with understanding is the following: the excitation by the laser must occur to the biexciton state $$| b \rangle$$. This happens through a virtual level (see image).

But, for this to happen it looks like 2 photons are stacked on top of each other. Can someone explain the process that allows this? So far when I heard about resonant excitation, it was always with single photons matching the "energy gap" between state 1 and state 2.

Addendum: so, while typing my question, I came across a passage in my book on quantum optics that mentions "two-photon absorption" from the ground state. I guess the question thus boils down to: how does two-photon absorption work?