The superconducting analog of the single-electron transistor (SET), the single-Cooper-pair transistor (SCPT), is comprised of two ultrasmall tunnel junctions in series forming an island (see Figure 1(a)). Transport through the SCPT depends on the electrostatic energy required to charge the island, as in the SET, but also on the Josephson coupling across the junctions. An SCPT was first demonstrated Fulton et al. in 1989, and interest has been renewed with recent work on SCPT-based qubits that have recently demonstrated coherence times on the order of 1 υs. In this paper, we address a long-standing question: What are the practical requirements for obtaining SCPT devices in which coherent transport of Cooper pairs is truly dominant, as required by various applications? We show that a controlled change in the spatial profile of the superconducting energy gap affects this transport dramatically.
Citation: Journal of the Physical Society of Japan
Pub Type: Journals
Cooper pair box, qubits, SET, superconductivity