Artificial Complexity: Quantum optics probes the interactions between light and matter. Building up from a simple, single-atom system, the exchange of virtual photons between systems of several (or many) atoms is expected to give rise to many exotic effects. Because controlling the separation of the atoms on the atomic scale is experimentally challenging, artificial atom systems may provide a more tractable route for systematic study, as described by van Loo et al. (p. 1494 , published online 14 November). Using a system of two separate superconducting qubits in a microwave transmission line, they show how the interaction between the two qubits can be controlled and mediated by electromagnetic modes. The results illustrate a feasible route to probing the complexity of many-body effects that may otherwise be difficult to realize.

We study the collective effects that emerge in waveguide quantum electrodynamics where several (artificial) atoms are coupled to a one-dimensional superconducting transmission line. Since single microwave photons can travel without loss for a long distance along the line, real and virtual photons emitted by one atom can be reabsorbed or scattered by a second atom. Depending on the distance between the atoms, this collective effect can lead to super- and subradiance or to a coherent exchange-type interaction between the atoms. Changing the artificial atoms transition frequencies, something which can be easily done with superconducting qubits (two levels artificial atoms), is equivalent to changing the atom-atom separation and thereby opens the possibility to study the characteristics of these collective effects. To study this waveguide quantum electrodynamics system, we extend previous work and present an effective master equation valid for an ensemble of inhomogeneous atoms driven by a coherent state. Using input-output theory, we compute analytically and numerically the elastic and inelastic scattering and show how these quantities reveal information about collective effects. These theoretical results are compatible with recent experimental results using transmon qubits coupled to a superconducting one-dimensional transmission line [van Loo et al. (unpublished)].

We experimentally demonstrate the in situ tunability of the minimum energy splitting (gap) of a superconducting flux qubit by means of an additional flux loop. Pulses applied via a local control line allow us to tune the gap over a range of several GHz on a nanosecond time scale. The strong flux sensitivity of the gap (up to $\sim 0.7\mathrm{GHz}/\mathrm{m}{\Phi }_0$) opens up the possibility to create different types of tunable couplings that are effective at the degeneracy point of the qubit. We investigate the dependence of the relaxation time and the Rabi frequency on the qubit gap.

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscillator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency $\nu${}osc we find the largest vacuum Rabi splitting of $\sim${}0.1$\nu${}osc. Here being at resonance coincides with the optimal coherence of the symmetry point.