1. An entanglement-build-path switch for selective routing of entanglement building, the switch comprising: physical hardware arranged to support a group of interface qubits and at least one intermediate qubit with an associated light-field emission and measurement arrangement; a selective state-capture arrangement for capturing the state of a selected one of the interface qubits by interaction with a light field; an optical merge arrangement for enabling a said light field that has interacted with any one of the interface qubits, to further interact with a said intermediate qubit and be measured whereby to entangle that intermediate qubit with the interface qubit; and a switch control for controlling the selective state-capture arrangement to separately entangle a first and a second interface qubit of said group with said at least one intermediate qubit and for causing the intermediate-qubit light-field emitter and measurement arrangement to entangle intermediate qubits with each other where there are multiple such qubits, and to remove any involved intermediate qubit from an entanglement.