Experimental characterization of an on-chip cascaded qubit chain (Part 1)

Unidirectional coupling between multiple superconducting qubits and a single waveguide produces a cascaded quantum system that can be used for quantum networking and studying fundamental quantum optics. To date, demonstrations of cascaded qubit-waveguide systems remain limited to small systems because of the complexity of realizing unidirectional qubit-photon interfaces. Here, we present experimental results demonstrating an array of 4 cascaded transmon qubits using on-chip unidirectional interfaces embedded in a metamaterial waveguide. Each qubit is parametrically coupled to two spatially separated points on the waveguide, and interference between radiation pathways mediates chiral atom-photon interactions. We study the elastic scattering response of the qubit chain, elucidating the role of coherent driving and waveguide decay on the power-dependent transmission. We then probe the cascaded system with weak coherent pulses and observe a Fock-state dependent group delay, consistent with theoretical predictions. Lastly, we discuss our ongoing efforts to probe entanglement stabilization in the qubit chain using a strong classical drive. Our results mark a step toward building scalable systems for studying chiral quantum optics.

Part 1 introduces the cascaded qubit platform and presents experimental confirmation of chirality and the cascading of the qubit chain.