Waveguide quantum electrodynamics with superconducting artificial atom and spin ensembles

Io-Chun Hoi

In this talk, I will address recent advances in waveguide quantum electrodynamics with superconducting artificial atom and spin ensembles.

In the first set of experiments, we investigate the amplification of a microwave probe signal by a superconducting artificial atom, a transmon, positioned at the end of a semi-infinite transmission line, under a strong pump field. Due to the weak anharmonicity of the artificial atom, the strong pump field creates multi-photon excitations among the dressed states. Transitions between these dressed states, Rabi sidebands, give rise to either amplification or attenuation of the weak probe. We obtain a maximum power amplification of 1.4, and near-quantum-limited added noise, due to quantum coherence between Rabi sidebands.

In the second set of experiments, we demonstrate two methods, both using just a single artificial atom, enabling dynamic control over microwave light velocities. Our methods are based on two distinct mechanisms harnessing the balance between radiative and non-radiative decay rates of a superconducting artificial atom in front of a mirror. In the first method, we tune the radiative decay of the atom using interference effects due to the mirror; in the second method, we pump the atom to control its non-radiative decay through the Autler-Townes effect. When the half the radiative decay rate exceeds the non-radiative decay rate, we observe positive group delay; conversely, dominance of the non-radiative decay rate results in negative group delay.

In the third set of experiments, we investigate microwave interference from a spin ensemble and its mirror image in a one-dimensional (1D) waveguide. Away from a mirror, the resonance frequency of the Kittel mode (KM) inside a ferrimagnetic spin ensemble has a sinusoidal frequency shift as the normalized distance increases compared to the setup without the mirror. This shift is attributed to the interaction of the KM with its own image. Meanwhile, its radiative decay rate shows a cosine squared oscillation.