Chiral coupling of a superconducting artificial atom to a 1-dimensional waveguide

Chiral light-matter interaction, wherein the coupling between atoms and photons depends on the propagation direction of light, enables a host of opportunities in quantum science. Unidirectional coupling of multiple atoms to a photonic bath produces a cascaded quantum system, with applications in deterministic state transfer, gate operations with itinerant photons, and study of quantum many-body physics. Here, we report on the realization of a superconducting artificial atom with unidirectional strong coupling to a waveguide. Our system operates in the giant atom regime of waveguide QED; it contains a transmon qubit with time-modulated couplings to two points of a microwave coplanar waveguide. Chirality is achieved by tailoring interference between these radiation pathways. In our device, coupling to forward propagating modes exceeds backward coupling by approximately two orders of magnitude. To demonstrate the quantum non-linear response of our system, we perform resonance fluorescence measurements on the chiral atom, observing Mollow triplets under a strong resonant drive. Further, we show that the chiral behavior can be extended to the higher energy levels of the transmon qubit, opening up a range of rich possibilities for engineering unidirectional light-matter gate operations or quantum state transfer. In the future, our device may serve as a fundamental building block for realizing cascaded quantum systems without the need for bulky, lossy circulators.