Quantum entanglement concentration for photonic polarization state assisted by electron spin
Authors: Zhao Rui-Tong  , Liang Rui-Sheng  , Wang Fa-Qiang  ,   Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, School for Information and Optoelectronic Science and Engineering, South China Normal University, Guangzhou 510006, China
Published: December 2020

In order to assure the security of the long-distance quantum communication, the maximum entangled state is necessary. However, the decoherence of the entanglement is inevitable because of the channel noise and the interference of the environment. Quantum entanglement concentration can be used to convert a non-maximum entangled state into a maximum one. In previous entanglement concentration proposals, we need the initial coefficients of non-maximum entangled state or repeat the entanglement concentration process to improve the possibility of success, which reduces the efficiency of the entanglement concentration. A more efficient entanglement concentration for phontonic polarization state is proposed in this paper, which is based on the interaction between circularly polarized light and quantum dot-cavity system. An auxiliary photon is introduced to connect two distant participants. To overcome the channel noise, the auxiliary photon transmits though two channels between the two participants. The photons interact with coupled quantum dot-cavity before and after the auxiliary photon transmission. Then the states of spins and auxiliary photon are measured, and the maximum phontonic polarization entangled state is obtained by single-photon operations according to the measurement results. The success possibility of the proposed scheme is 1 in ideal conditions, that is, the concentration can be realized deterministically. However, the cavity leakage is unavoidable, so the fidelity of the entanglement concentration is calculated by taking one of the measurement results for example. The results show that the influences of the initial coefficients of non-maximum entangled state on the fidelity can be ignored in most cases, which saves a mass of photons used to measure the initial coefficients of the non-maximum entangled state. The fidelities with varying coupling strengths and cavity leakages are also shown in the paper. In the case of weak coupling, the fidelity is low and varies sharply with cavity leakage. Fortunately, the fidelity will plateau in a strong coupling case, and reaches 99.8% with a coupling strength 0.7 for diverse cavity leakages. Much progress has been made in the study of the strong coupling between quantum dot and optical cavity, which can satisfy the requirement of our entanglement concentration. So the proposed scheme is feasible in the current experimental conditions. In general, our proposal still maintains high fidelity even considering the cavity leakage, and the initial information about partially entangled state and the repetition of the entanglement concentration process are not required. This not only improves the security of the quantum entanglement concentration, but also contributes to efficient quantum information processing with less quantum resources. These characteristics increase the universality and efficiency of the entanglement concentration, thus assuring the quality of the long-distance quantum entanglement.