Entanglement of two superconducting qubits in a waveguide cavity via   monochromatic two-photon excitation
  We report a system where fixed interactions between non-computational levels make bright the otherwise forbidden two-photon 00 --> 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 90% (unconstrained) and 86% (maximum likelihood estimator). 
S. Poletto, Jay M. Gambetta, Seth T. Merkel, John A. Smolin, Jerry M.   Chow, A. D. Corcoles, George A. Keefe, Mary B. Rothw...
quant-ph cond-mat.supr-con