  Entanglement is the fuel of advanced quantum technology. It is for instance
consumed in measurement-based quantum computing and allows loss-tolerant
encoding of quantum information. In photonics, entanglement has traditionally
been generated probabilistically, requiring massive multiplexing for scaling up
to many photons. An alternative approach utilizes quantum emitters in
nanophotonic devices for deterministic generation of single photons, which an
be extended to two- and multi-photon generation on demand. The proposed
polarization-entanglement sources are, however, incompatible with spatial
dual-rail qubit encoding, which is preferred in photonic quantum computing
realized in scalable integrated photonic circuits. Here we propose and
experimentally realize an on-demand source of dual-rail photon pairs using a
quantum dot in a planar nanophotonic waveguide. The source exploits the
cascaded decay of a biexciton state and chiral light-matter coupling to achieve
deterministic generation of spatial dual-rail Bell pairs with the amount of
entanglement determined by the chirality. The operational principle can readily
be extended to multi-photon entanglement generation, and such sources may be
interfaced with advanced photonic-integrated circuits, e.g., for efficient
preparation of entanglement resource states for photonic quantum computing.
