Chiral cavity quantum electrodynamics

Cavity quantum electrodynamics, which explores the granularity of light by coupling a resonator to a nonlinear emitter1, has played a foundational role in the development of modern quantum information science and technology. In parallel, the field of condensed matter physics has been revolutionized by the discovery of underlying topological2–4, often arising from the breaking of time-reversal symmetry, as in the case of the quantum Hall effect. In this work, we explore the cavity quantum electrodynamics of a transmon qubit in a topologically nontrivial Harper–Hofstadter lattice5. We assemble the lattice of niobium superconducting resonators6 and break time-reversal symmetry by introducing ferrimagnets7 before coupling the system to a transmon qubit. We spectroscopically resolve the individual bulk and edge modes of the lattice, detect Rabi oscillations between the excited transmon and each mode and measure the synthetic-vacuum-induced Lamb shift of the transmon. Finally, we demonstrate the ability to employ the transmon to count individual photons8 within each mode of the topological band structure. This work opens the field of experimental chiral quantum optics9, enabling topological many-body physics with microwave photons 10,11 and providing a route to backscatter-resilient quantum communication.