Chiral cavity quantum electrodynamics

Cavity quantum electrodynamics, which explores the granularity of light by coupling a resonator to a nonlinear emitter 1 , 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 topological 2 – 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 lattice 5 . We assemble the lattice of niobium superconducting resonators 6 and break time-reversal symmetry by introducing ferrimagnets 7 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 photons 8 within each mode of the topological band structure. This work opens the field of experimental chiral quantum optics 9 , enabling topological many-body physics with microwave photons 10 , 11 and providing a route to backscatter-resilient quantum communication.