  The strong coupling limit of cavity quantum electrodynamics (QED) implies the
capability of a matter-like quantum system to coherently transform an
individual excitation into a single photon within a resonant structure. This
not only enables essential processes required for quantum information
processing but also allows for fundamental studies of matter-light interaction.
In this work we demonstrate strong coupling between the charge degree of
freedom in a gate-detuned GaAs double quantum dot (DQD) and a frequency-tunable
high impedance resonator realized using an array of superconducting quantum
interference devices (SQUIDs). In the resonant regime, we resolve the vacuum
Rabi mode splitting of size $2g/2\pi = 238$ MHz at a resonator linewidth
$\kappa/2\pi = 12$ MHz and a DQD charge qubit dephasing rate of $\gamma_2/2\pi
= 80$ MHz extracted independently from microwave spectroscopy in the dispersive
regime. Our measurements indicate a viable path towards using circuit based
cavity QED for quantum information processing in semiconductor nano-structures.
