Quantum repeater scheme from refs. 13 and 14 (CTSL scheme). Quantum repeater scheme from refs. 13 and 14 (CTSL scheme). (a) This scheme has exactly two qubits per node. The communication qubits (green nodes) are used for entanglement generation and short-term storage; the storage qubits (black nodes) are used for long-term storage. (b) With the help of local gates (orange solid ovals) between communication and storage qubits, the entangled state between communication qubits can be used to implement entangling gates (e.g., the Controlled NOT gate) between storage qubits from neighboring nodes. The effective remote gate is highlighted by the orange dashed oval. Such remote gate is sufficient for entanglement connection and purification of storage qubits. The communication qubits are omitted in the following plots. (c and d) Entanglement connection to produce an unpurified entangled state with distance n. (e–g) Entanglement connection to produce an unpurified entangled state with distance n − 2 to purify the entangled state with distance n. (f′ and f ″) Illustration of multilevel pumping. An entangled state with distance n − 4 is used to purify an entangled state with distance n − 2, and the (purified) latter is used to purify an entangled state with distance n. The only difference between f and f″ is the fidelity of the entangled state with distance n − 2. The latter has higher fidelity. Liang Jiang et al. PNAS 2007;104:44:17291-17296 ©2007 by National Academy of Sciences