Superconducting Thin-Films for Quantum Devices with Off-Line Quality Assessment

Quantum computers are capable of high-speed calculations, far superior to modern supercomputers. The data of each strand of the calculation does not need to be stored permanently, but challenges include maintaining the integrity of the data (coherence) for long enough for calculations to be performed with a low error rate. To increase coherence times to useful values for multi-qubit systems, research has focussed on the design of resonators and qubits, often overlooking the design of the materials used [1]. Smart materials selection and control of the chemistry and microstructure to optimise the Q-factor of the resonators (ratio of stored energy to input energy) and increase coherence times can help make superconductor-based quantum computers more viable.<br/>Most materials used in quantum circuitry are selected for ease and reliability of deposition of multi-layered JJ structures with reasonable superconducting properties. However, it is known that other materials may have better properties if they could be optimised. For example, control of deposition flux can affect the surface roughness of a material, a parameter that can increase surface resistance and reduce the Q-factor of a quantum resonator. The structure and chemistry of the material can also impact superconducting properties. For example, changes in crystal structure, degree of crystallinity, crystallographic texture and chemical composition can affect the critical temperature of the superconductor of the film. A systematic study of carefully controlled deposition properties, linking film growth and superconducting properties, would be invaluable for assessing alternative materials for quantum resonators.<br/>In this presentation, the results of a thorough, systematic study of the growth of superconducting materials of merit for use in quantum circuits will be reported. This study links growth parameters, material properties and superconducting performance. Initially, metal thin films- specifically niobium and molybdenum - have been deposited using magnetron sputtering, with the aim of fully understanding the deposition window before moving on to more complex materials, such as alloys and nitrides. The thin films have been analysed using SEM, XRD, XPS and AFM to determine microstructural and crystallographic properties. Electrical transport measurements have been performed to determine room temperature resistivity, transition temperature and transition width, critical field and critical current. We also aim to present preliminary low temperature microwave measurements on resonator devices.<br/>Conditions for the growth of high-quality superconducting films will be identified and this presentation will explore any links between deposition properties and material properties, to assess whether high quality room temperature and low temperature properties can be used to predict ultra-low temperature performance of materials for quantum devices.<br/>1. Oliver, W.D. and P.B. Welander, <i>Materials in superconducting quantum bits.</i> MRS Bulletin, 2013. <b>38</b>(10): p. 816-825.