Engineering light-matter interactions at the quantum level has been central
to the pursuit of quantum optics for decades. Traditionally, this has been done
by coupling emitters, typically natural atoms and ions, to quantized
electromagnetic fields in optical and microwave cavities. In these systems, the
emitter is approximated as an idealized dipole, as its physical size is orders
of magnitude smaller than the wavelength of light. Recently, artificial atoms
made from superconducting circuits have enabled new frontiers in light-matter
coupling, including the study of "giant" atoms which cannot be approximated as
simple dipoles. Here, we explore a new implementation of a giant artificial
atom, formed from a transmon qubit coupled to propagating microwaves at
multiple points along an open transmission line. The nature of this coupling
allows the qubit radiation field to interfere with itself leading to some
striking giant-atom effects. For instance, we observe strong
frequency-dependent couplings of the qubit energy levels to the electromagnetic
modes of the transmission line. Combined with the ability to in situ tune the
qubit energy levels, we show that we can modify the relative coupling rates of
multiple qubit transitions by more than an order of magnitude. By doing so, we
engineer a metastable excited state, allowing us to operate the giant transmon
as an effective lambda system where we clearly demonstrate electromagnetically
induced transparency.Comment: 12 pages, 8 figure