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

Abstract

Micrometre-scale superconducting circuits can act as quantum two-level systems, but unlike in their natural counterparts—such as atoms—the parameters of these ‘artificial qubits’ can be controlled externally. This tunability has now been used to break the symmetry of the system hamiltonian in a controlled manner. Superconducting qubits1,2 behave as artificial two-level atoms and are used to investigate fundamental quantum phenomena. In this context, the study of multiphoton excitations3,4,5,6,7 occupies an important role. Moreover, coupling superconducting qubits to onchip microwave resonators has given rise to the field of circuit quantum electrodynamics8,9,10,11,12,13,14,15 (QED). In contrast to quantum-optical cavity QED (refs 16, 17, 18, 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 qubit20 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 property21. Our study provides fundamental insight into the interplay of multiphoton processes and symmetries in a qubit–resonator system.