A method and apparatus (80) are provided for routing entanglement building between a selected pairing of interface qubits (82). The qubits of the selected pairing of interface qubits (82) are separately entangled with at least one intermediate qubit (84) by interacting respective light fields with the interface qubits of the selected pairing and using an optical merge arrangement (83) to further interact the light fields with at least one intermediate qubit (84). Where there are multiple intermediate qubits (84) the intermediate qubits are entangled with each other. The or each entangled intermediate qubit (84) is then removed from entanglement.