In this case, the quantum network consists of many short distance links of perhaps tens or hundreds of kilometers. In the simplest case of a single repeater, two pairs of entangled qubits are established: | A ⟩ ⟩ and | R a ⟩ ⟩ located at the sender and the repeater, and a second pair | R b ⟩ ⟩ and | B ⟩ ⟩ located at the repeater and the receiver. These initial entangled qubits can be easily created, for example through parametric down conversion, with one qubit physically transmitted to an adjacent node. At this point, the repeater can perform a Bell measurement on the qubits | R a ⟩ ⟩ and | R b ⟩ ⟩ thus teleporting the quantum state of | R a ⟩ ⟩ onto | B ⟩ ⟩ . This has the effect of "swapping" the entanglement such that | A ⟩ ⟩ and | B ⟩ ⟩ are now entangled at a distance twice that of the initial entangled pairs. It can be seen that a network of such repeaters can be used linearly or in a hierarchical fashion to establish entanglement over great distances.