The illustration shows an optical chip embedded with a so-called quantum dot. The quantum dot can function as a gateway or transistor for photons. The drawing on top shows that if you send a single photon into the quantum dot, it will be thrown back -- the gateway is closed. The lower drawing shows that if you send two photons, the gateway opens the two photons are sent onwards. So the quantum dot works as a photon contact and this is an important component when you want to build complex quantum photonic circuits on a large scale.