Figure 4

Schematic of building blocks for quantum network. (a) Two designs of building blocks for quantum networks: (Top) For one-way communication to transmit quantum information along the network, in which the qubit carrying information travels L 0 between repeater nodes. The other qubit is staying in the repeater. Then, CBM is performed on the transmitted and stationary qubits. (Bottom) For entanglement distribution between remote places, in which CBM is performed to link the entangled pairs | Φ + 〉 from adjacent nodes. Each qubit travels L 0 / 2 to meet in the middle before CBM. Note that both designs of building blocks yield the same success probabilities, P s ( η L 0 , η 0 ) = P s ( η L 0 / 2 , η L 0 / 2 ) and cost the same number of photons on average. (b) A quantum repeater for one-way communication is composed of two parts: the preparation of entangled pair | Φ + 〉 and CBM on two qubits (one is received from the previous node and the other from | Φ + 〉 ). The other qubit of | Φ + 〉 is transmitted to the next node. The result of CBM is directly sent to Bob via classical communication based on which the transmitted information can be recovered at the final step. Losses during preparation and measurement process also affect the performance. The effective loss rate of photons inside of the repeater is referred as η 0 , estimated with source efficiency ε s , detector efficiency ε d , and the loss during the generation time of | Φ + 〉 .