Markus Schmid «  Georg Reuther »  Sebastian Deffner
Georg Reuther
Decoherence and Time-Resolved Readout in Superconducting Quantum Circuits
Supervisor: Prof. Dr. Sigmund Kohler [Theoretical physics I]
Date of oral examination: 02/10/2011
157 pages, english , Logos-Verlag Berlin, 2011
ISBN: 978-3-8325-2846-1
Superconducting quantum circuits, being macroscopic quantum objects, are promising candidates for solid-state based quantum computation. Apart from good coherence, they feature enhanced controllability and offer various options of design and mutual coupling. In spite of all experimental progress made, however, the implementation of a scalable quantum computer remains a tough challenge due to uncontrolled interactions with external environments, causing decoherence. With a view to building more complex circuits, the first main part of this thesis deals with a dissipative superconducting circuit of two resonators coupled via a flux qubit. In this ``quantum switch'', the qubit acts as a tunable coupler between the resonators, which enables switching the interaction between the resonators on and off and, thus, allows generating two-resonator entanglement. A fully analytic dissipative theory shows that, even if the qubit has no dynamics, qubit relaxation affects the resonators to a non-negligible degree, whereas the impact of qubit dephasing can be discarded. These results provide a basis for a general formalism of decoherence and readout in multipartite quantum circuits with tunable coupling strengths. Another crucial requirement for quantum information processing is the readout of a qubit state after performing gate operations. At the same time, it is desirable to demonstrate the coherence of qubit time evolution explicitly. In the second main part of this thesis we present a novel scheme for a time-resolved single-run measurement of coherent qubit dynamics with good measurement fidelity and small backaction. For a charge qubit probed by a weak high-frequency signal via a transmission line, we find that the reflected outgoing signal possesses a phase shift that is proportional to a qubit observable. A similar protocol relies on coupling a flux qubit to a resonantly driven high-frequency oscillator. Here, the resonance frequency of the oscillator depends on the qubit state. Thus, the oscillator response exhibits a small time-dependent phase shift, which again relates to the qubit dynamics. The experimental implementation of these schemes will permit the demonstration of quantum coherence of solid-state qubits in single-shot experiments.