  Recent technological advances in cavity quantum electrodynamics (CQED) are
paving the way to utilise multiple quantum emitters confined in a single
optical cavity. In such systems it is crucially important to control the
quantum mechanical coupling of individual emitters to the cavity mode. In this
regard, combining ion trap technologies with CQED provides a particularly
promising approach due to the well-established motional control over trapped
ions. Here we experimentally demonstrate coupling of up to five trapped ions in
a string to a high-finesse optical cavity. By changing the axial position and
spacing of the ions in a fully deterministic manner, we systematically
characterise their coupling to the cavity mode through visibility measurements
of the cavity emission. In good agreement with the theoretical model, the
results demonstrate that the geometrical configuration of multiple trapped ions
can be manipulated to obtain optimal cavity coupling. Our system presents a new
ground to explore CQED with multiple quantum emitters, enabled by the highly
controllable collective light-matter interaction.
