  Models of light-matter interactions typically invoke the dipole
approximation, within which atoms are treated as point-like objects when
compared to the wavelength of the electromagnetic modes that they interact
with. However, when the ratio between the size of the atom and the mode
wavelength is increased, the dipole approximation no longer holds and the atom
is referred to as a "giant atom". Thus far, experimental studies with
solid-state devices in the giant-atom regime have been limited to
superconducting qubits that couple to short-wavelength surface acoustic waves,
only probing the properties of the atom at a single frequency. Here we employ
an alternative architecture that realizes a giant atom by coupling small atoms
to a waveguide at multiple, but well separated, discrete locations. Our
realization of giant atoms enables tunable atom-waveguide couplings with large
on-off ratios and a coupling spectrum that can be engineered by device design.
We also demonstrate decoherence-free interactions between multiple giant atoms
that are mediated by the quasi-continuous spectrum of modes in the waveguide--
an effect that is not possible to achieve with small atoms. These features
allow qubits in this architecture to switch between protected and emissive
configurations in situ while retaining qubit-qubit interactions, opening new
possibilities for high-fidelity quantum simulations and non-classical itinerant
photon generation.
