  Routing quantum information between non-local computational nodes is a
foundation for extensible networks of quantum processors. Quantum information
transfer between arbitrary nodes is generally mediated either by photons that
propagate between them, or by resonantly coupling nearby nodes. The utility is
determined by the type of emitter, propagation channel, and receiver.
Conventional approaches involving propagating microwave photons have limited
fidelity due to photon loss and are often unidirectional, whereas architectures
that use direct resonant coupling are bidirectional in principle, but can
generally accommodate only a few local nodes. Here we demonstrate
high-fidelity, on-demand, directional, microwave photon emission. We do this
using an artificial molecule comprising two superconducting qubits strongly
coupled to a bidirectional waveguide, effectively creating a chiral microwave
waveguide. Quantum interference between the photon emission pathways from the
molecule generates single photons that selectively propagate in a chosen
direction. This circuit will also be capable of photon absorption, making it
suitable for building interconnects within extensible quantum networks.
