Measurement Back-Action and Photon Detection in Microwave Quantum Optics

Licentiate thesis, 2012

In quantum optics, the interaction between atoms and photons is studied. In recent years, microwave quantum optics with superconducting circuits has emerged as an important tool for fundamental quantum optics experiments, and also as a promising way for implementing quantum computation. The main reason for this development is the ease with which strong coupling and other properties of artificial atoms and microwave photons can be engineered in such a setup. This thesis is comprised of two papers dealing with measurements in microwave quantum optics. In Paper I, we show how unwanted measurement back-action can be undone for certain measurements on one and two qubits dispersively coupled to a microwave resonator. An important application of this result is to improve parity measurements, which are integral to error-correction codes needed to implement large-scale quantum computing. In Paper II, we investigate the possibility of using a three-level artificial atom, a transmon, to mediate a cross-Kerr type interaction between photons. The idea is to use it as a single-photon detector in the microwave regime, a component currently missing in the experimentalist's toolbox. We show that there are fundamental limitations to this setup, resulting in an unsatisfactory signal-to-noise ratio.