Qubit-controlled directional edge states in waveguide QED*

We propose an in-situ tunable chiral quantum system, composed of a quantum emitter coupled to a waveguide based on the Rice-Mele model (where we alternate both the on-site potentials and tunnel couplings between sites in the waveguide array). Specifically, we show that the chirality of photonic bound state, that emerges in the bandgap of the waveguide, depends only on the energy of the qubit; a parameter that is easy to tune in many artificial atoms. In contrast to previous proposals that have either shown imperfect chirality or fixed directionality, our waveguide quantum electrodynamics scheme achieves both perfect chirality and the capability to switch the directionality on demand with just one tunable element in the device. We implemented our model on superconducting circuit and demonstrated the directionality by measuring microwave signal transmissions and refelctions. The results show technological promise in creating long-range couplers between qubits while maintaining, in principle, zero crosstalk.