Design aspects of superconducting-phase quantum bits - art. no. 174511

A superconducting-phase quantum bit (qubit) involves three or more Josephso n junctions combined into a superconducting loop and defines one of the pro mising solid-state device implementations for quantum computing. Recently, so called pi junctions, Josephson junctions with a ground state characteriz ed by a pi -phase shift across, have attracted much attention. We show how to make use of such pi junctions in the construction of superconducting pha se qubits and discuss the advantage over conventional designs based on magn etically frustrated loops. Starting from a basic five-junction loop with on e pi junction, we show how to construct effective junctions with degenerate minima characterized by phase shifts 0 and pi and superconducting-phase sw itches. These elements are then combined into a superconducting-phase qubit which operates exclusively with switches, thus avoiding permanent contact with the environment through external biasing. The resulting superconductin g-phase qubits can be understood as the macroscopic analog of the ''quiet'' s-wave-d-waves-wave Josephson-junction qubits introduced by Ioffe et al. [ Nature (London) 398, 679 (1999)].