  We have studied interactions between two capacitively coupled GaAs/AlGaAs
few-electron double quantum dots. Each double quantum dot defines a tunable
two-level system, or qubit, in which a single excess electron occupies either
the ground state of one dot or the other. Applying microwave radiation we
resonantly drive transitions between states and non-invasively measure
occupancy changes using proximal quantum point contact charge detectors. The
level structure of the interacting two-qubit system is probed by driving it at
a fixed microwave frequency whilst varying the energy detuning of both double
dots. We observe additional resonant transitions consistent with a simple
coupled two-qubit Hamiltonian model.
