Two-photon probe of the Jaynes-Cummings model and controlled symmetry breaking in circuit QED

Superconducting qubits(1,2) behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multiphoton excitations(3-7) occupies an important role. Moreover, coupling superconducting qubits to onchip microwave resonators has given rise to the field of circuit quantum electrodynamics(8-15) (QED). In contrast to quantum-optical cavity QED (refs 16-19), circuit QED offers the tunability inherent to solid-state circuits. Here, we report on the observation of key signatures of a two-photon-driven Jaynes-Cummings model, which unveils the upconversion dynamics of a superconducting flux qubit(20) coupled to an on-chip resonator. Our experiment and theoretical analysis show clear evidence for the coexistence of one-and two-photon-driven level anticrossings of the qubit-resonator system. This results from the controlled symmetry breaking of the system hamiltonian, causing parity to become a not-well-defined property(21). Our study provides fundamental insight into the interplay of multiphoton processes and symmetries in a qubit-resonator system.