  We demonstrate a superconducting artificial atom with strong unidirectional
coupling to a microwave photonic waveguide. Our artificial atom is realized by
coupling a transmon qubit to the waveguide at two spatially separated points
with time-modulated interactions. Direction-sensitive interference arising from
the parametric couplings in our scheme results in a non-reciprocal response,
where we measure a forward/backward ratio of spontaneous emission exceeding
100. We verify the quantum nonlinear behavior of this artificial chiral atom by
measuring the resonance fluorescence spectrum under a strong resonant drive and
observing well-resolved Mollow triplets. Further, we demonstrate chirality for
the second transition energy of the artificial atom and control it with a pulse
sequence to realize a qubit-state-dependent non-reciprocal phase on itinerant
photons. Our demonstration puts forth a superconducting hardware platform for
the scalable realization of several key functionalities pursued within the
paradigm of chiral quantum optics, including quantum networks with all-to-all
connectivity, driven-dissipative stabilization of many-body entanglement, and
the generation of complex non-classical states of light.
