Quantum optics with artificial atoms

Abstract

Cover: Clockwise from top left: a setup for microwave photon detection using cascaded three-level systems (Paper V); time evolution of the coherent state in a resonator, dispersively coupled to a qubit, conditioned on the qubit state (Paper I); a model for a giant multi-level atom coupled at several points (possibly wavelengths apart) to a 1D waveguide (Paper VII); zoom-in on part of a transmon qubit coupled to surface acoustic waves, realizing a giant atom (Paper VI). AbstractQuantum optics is the study of interaction between atoms and photons. In the eight papers of this thesis, we study a number of systems where artificial atoms (here, superconducting circuits emulating the level structure of an atom) enable us to either improve on known concepts or experiments from quantum optics with natural atoms, or to explore entirely new regimes which have not been possible to reach in such experiments.Paper I shows how unwanted measurement back-action in a parity measurement can be avoided by fully using the information in the measurement record. Paper III is a proof-of-principle experiment demonstrating that an artificial atom built from superconducting circuits can mediate a strong photon-photon interaction. In Papers II and V, we theoretically investigate whether this interaction can be used in a setup for detecting propagating microwave photons, making the photon to be detected impart a phase shift on a coherent probe signal. We find that one atom is not enough to overcome the quantum background noise, but it turns out that several atoms cascaded in the right way can do the trick.In Paper IV, we explain experimental results for a driven artificial atom coupled to photons in a resonator. The last three papers all deal with an artificial atom coupled to a bosonic field at several points, which can be wavelengths apart. Paper VI is a ground-breaking experimental demonstration of coupling between an artificial atom and propagating sound in the form of surface acoustic waves (SAWs). The short SAW wavelength makes the atom "giant" in comparison; the effects of this new regime is explored theoretically in Paper VII, where the multiple coupling points are shown to give interference effects affecting both the atom's relaxation rate and its energy levels. In Paper VIII, an artificial atom in front of a mirror is used to probe the mode structure of quantum vacuum fluctuations.