Engineering superconducting qubits
The very features that make quantum mechanics so weird and wonderful, when compared to everyday experience of the classical world of computing, are those that also underpin its potentially revolutionary applications. A new quantum information technology could emerge in the future, based on research in the field of information processing in accordance with its fundamental laws. Intensive research within the framework of the SQUBIT-2 project aimed at manipulating quantum information using solid state systems and more specifically, superconducting electronic circuits. Most information manipulation is currently done digitally and data is processed and stored as bits. The two states of a conventional bit may take different forms, such as two different voltages across a transistor on a chip. Recent developments in nanotechnology have, however, allowed the design of superconducting circuits with small tunnel junctions, offering the possibility of manipulating single Cooper pairs and building quantum bits. The energy levels of qubits result from the interplay between the charging energy of a single Cooper pair and energy characterising the tunnelling of Cooper pairs across Josephson junctions. Utilisation of niobium (Nb) in ultra-small tunnel junctions has been the challenge undertaken by the SQUBIT-2 project partners at the Physikalisch-Technische Bundesanstalt in Germany. The use of niobium instead of the conventionally used aluminium (Al) appeared to be more attractive. It was proven to provide for an extension in the working frequency of quantum computing devices built on superconducting circuits with Nb-based tunnel junctions. Josephson junctions with linear dimensions as small as 70nm were fabricated from the standard trilayer of Nb/Al-AlOx/Nb. To overcome limitations of the standard shadow evaporation technique used for the fabrication of submicron Al-based tunnel junctions, an alternative fabrication process was proposed and optimised. Electron beam lithography, dry etching, anodisation and planarisation by chemical-mechanical polishing, were combined to fabricate tunnel junctions of high quality. These complied with the desired relation between the charging energy of Cooper pairs and Josephson coupling energy. Further research has been programmed to also improve the characteristic coherence times of qubits build on Nb-based junctions. Figure caption: SEM micrograph showing intermediate fabrication step of a device containing four sub-micrometre Nb/AlOx//Nb junctions.
