Conversion of Knill–Laflamme–Milburn Entanglement to Greenberger–Horne–Zeilinger Entanglement in Decoherence‐Free Subspace

In the process of quantum information processing, decoherence effect caused by the coupling between system and its environment will no doubt lead to the error of quantum information stored in the system. To decrease the influence of decoherence effect, decoherence‐free subspace (DFS) is introduced. In this work, several schemes to convert the polarized‐entangled Knill–Laflamme–Milburn (KLM) states into Greenberger–Horne–Zeilinger (GHZ) states in DFS by using the weak cross‐Kerr nonlinearity are proposed. Numerical analysis shows that the proposed schemes have high fidelity and success probability. Due to the unique applications of multi‐qubit quantum entangled states in quantum information processing and the robust feature of DFS, the proposed schemes may play positive roles in the future development of quantum communication and quantum information processing tasks. Several schemes to convert the polarized‐entangled Knill–Laflamme–Milburn states into Greenberger–Horne–Zeilinger states in decoherence‐free subspace (DFS) by using the weak cross‐Kerr nonlinearity are proposed. DFS is introduced to decrease the influence of decoherence effect. Numerical analysis shows that the proposed schemes have high fidelity and success probability.