Realizing giant artificial atoms in superconducting waveguide QED

In most studies of light-matter interaction, the atoms, either natural or artificial, are approximated as featureless dipoles, since the atomic dimension is much smaller than the wavelength of light. However, a new regime in waveguide QED, first proposed by Kockum et al, can realize a “giant” artificial atom by coupling to light at multiple points along a waveguide. As a result, the atom interacts with itself, resulting in a range of phenomena including non-Markovian dynamics and frequency-dependent coupling. The same proposal also discussed possibilities to extend this architecture to multiple giant atoms with interesting new physics. Motivated by this, we experimentally investigate circuits with one and two giant transmon qubits which are coupled to propagating microwaves at multiple points separated by wavelength-scale distances. For one qubit circuit, we demonstrate that we can enhance or suppress the relaxation rate of the 1-2 transition relative to the 0-1 transition by more than an order of magnitude. Using this capability, we show that we can engineer the giant transmon into an effective lambda system, including demonstrating EIT. We will also present preliminary measurements of a circuit with two giant qubits coupled in a braided geometry.