Entanglement of Two Superconducting Qubits in a Waveguide Cavity via Monochromatic Two-Photon Excitation

We report a system where fixed interactions between noncomputational levels make bright the otherwise forbidden two-photon vertical bar 00 > -> vertical bar 11 > transition. The system is formed by hand selection and assembly of two discrete component transmon- style superconducting qubits inside a rectangular microwave cavity. The application of a monochromatic drive tuned to this transition induces two-photon Rabi-like oscillations between the ground and doubly excited states via the Bell basis. The system therefore allows all-microwave two-qubit universal control with the same techniques and hardware required for single qubit control. We report Ramsey-like and spin echo sequences with the generated Bell states, and measure a two-qubit gate fidelity of F-g = 90% (unconstrained) and 86% (maximum likelihood estimator). DOI: 10.1103/PhysRevLett.109.240505