Rectangular waveguide QED with superconducting qubits, PhD thesis

Coupling superconducting artificial atoms to the continuum of modes in a waveguide opens a door for probing a vast range of atom-light interactions in an open space configuration. In this thesis, we experimentally implement a platform where two transmon-type superconducting qubits are coupled to the one-dimensional fields confined in a rectangular waveguide.

Using this platform, we first show how field-mediated interactions between the atoms can be used to generate sub- and super-radiant states, which, when combined with the nonlinear saturability of superconducting qubits, can be exploited to experimentally implement a device whose light scattering properties depend on the driving direction, thus achieving nonreciprocal transmission of microwave radiation. This study paves the way towards the implementation of an integrated, low-loss microwave isolator, capable of coexisting with the technologies involved in superconducting circuits. Furthermore, it showcases the simplest possible implementation of a nonreciprocal device, as a minimum of two quantum emitters is needed to break the spatial symmetry of the configuration.

Using the same platform, we then show how long-lived subradiant states can be also exploited to stabilize entanglement between two superconducting qubits. We propose an experimental implementation, discuss its experimental feasibility and propose mechanisms to certify entanglement between the qubits.

The characteristic cutoff of the rectangular waveguide can be exploited to reach a different atom-field coupling regime. We study this regime and experimentally show how it leads to localized atom-photon bound states in the stopband of the waveguide. We show evidence of photon localization in our system, as well as a tunable interaction between two bound states mediated by the evanescent, localized fields.

Finally, as a follow up of our investigations on nonreciprocal transmission of light, we discuss an experimental implementation of an on-chip three-port microwave circulator. The presented device relies on Aharonov-Bohm interference effects arising in a magnetic flux biased ring consisting of three Josephson junctions.