  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).
