Andreev Interferometry of Flux Qubits Driven By Radio Frequency Field

Abstract

In this thesis we present the continuing work done examining a system in which an Andreev interferometer is used to probe the state of a flux qubit. In particular, we show that the back action of the interferometer on the qubit is low enough that an energy gap can still be observed in the qubit, and present the first experimental evidence of resonant excitation of a flux qubit detected using an Andreev interferometer. We begin by discussing the theory of flux qubits and Andreev interferometers individually. We then go on to examine what happens when with these two types of structures are combined, with particular attention being paid to the consequences for the coherence time of the qubit. We then discuss the practical elements of the experiment, notably the development of a tri-layer resists system that can be used to create high quality mesoscopic structures. We present the experimental results, which show the evidence for resonant excitation of a qubit detected using an Andreev interferometer. The quality of these resonances suggests that the system has a coherence time of less than 1ns. To conclude we examine some ways in which we believe the system can be improved in order to allow more detailed spectroscopic and time resolved measurements.