  A scalable optical quantum information processor is likely to be a waveguide
circuit with integrated sources, detectors, and either deterministic
quantum-logic or quantum memory elements. With microsecond coherence times,
ultrafast coherent control, and lifetime-limited transitions, semiconductor
quantum-dot spins are a natural choice for the static qubits. However their
integration with flying photonic qubits requires an on-chip spin-photon
interface, which presents a fundamental problem: the spin-state is measured and
controlled via circularly-polarised photons, but waveguides support only linear
polarisation. We demonstrate here a solution based on two orthogonal photonic
nanowires, in which the spin-state is mapped to a path-encoded photon, thus
providing a blue-print for a scalable spin-photon network. Furthermore, for
some devices we observe that the circular polarisation state is directly mapped
to orthogonal nanowires. This result, which is physically surprising for a
non-chiral structure, is shown to be related to the nano-positioning of the
quantum-dot with respect to the photonic circuit.
