Figure 1
From: Photonic ququart logic assisted by the cavity-QED system
Schematic dipole spin-dependent transitions with circularly polarized photons.
(a) A charged quantum dot inside a one-side micropillar microcavity interacting with circularly polarized photons. 
and 
are the input and output field operators of the waveguide, respectively. (b) The optical selection rules due to the Pauli exclusion principle. L and R denote the left and right circular polarization respectively. ↑ and ↓ represent the spins of the excess electron. ↑↓⇑ and ↓↑⇓ denote the negatively charged excitons.
