Fig. 1. Spin-dependent transitions for negatively charged exciton X−. (a) A charged QD inside a micropillar microcavity with circular cross section. (b) The spin selection rule for optical transitions of negatively charged exciton X− due to the Pauli’s exclusion principle. L and R represent the left- and the right-hand circularly polarized lights, respectively.

Fig. 2. Diagram of information transfer from a photon to a solid-state qubit. HWPs denote half-wave plates that are used to perform Hadamard operations. P denotes a π/2 phase shift on the |L〉a polarization state. PBS denotes a polarization beam splitter that transmits |R〉a photon and reflects |L〉a photon. D1 and D2 are single-photon detectors.

Fig. 3. Setup of the information transfer from a solid-state qubit to a photon. U^i is used to modulate the spin state of the QD. The modulation of the spin state of the QD will be implemented two times before (U^1) and after (U^2) the photon passes through the QD.

Fig. 4. Diagram of the information transfer between QD spins. Obviously, the red block shows the information transfer from a QD-spin qubit to a photon in Fig. 3, and the green block denotes the information transfer from a photon to a QD-spin qubit in Fig. 2.

Fig. 5. Diagram of the information transfer between photons. Photons a1 and a2 pass through the QD in sequence. U^i is also used to modulate the spin state of the QD. The modulation of the spin state of the QD will be implemented two times before (U^1) and after (U^2) the photon a2 passes through the QD.

Fig. 6. Fidelities of the information transfer schemes versus the coupling strength g/(κs+κ) for different leakage rates κs/κ. (a) The fidelity of the information transfer from a photon to a QD spin, which is equal to that from a QD spin to a photon. (b) The fidelity of the information transfer between two photons or two QD spins. Here, the four curves correspond to the cases of κs/κ=0, 0.2, 0.5, and 1, respectively. We take ωX−=ωc and γ=0.1κ.