Design aspects of superconducting-phase quantum bits

A superconducting-phase quantum bit (qubit) involves three or more Josephson junctions combined into a superconducting loop and defines one of the promising solid-state device implementations for quantum computing. Recently, so called pi junctions, Josephson junctions with a ground state characterized 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 phase qubits and discuss the advantage over conventional designs based on magnetically frustrated loops. Starting from a basic five-junction loop with one pi junction, we show how to construct effective junctions with degenerate minima characterized by phase shifts 0 and pi and superconducting-phase switches. 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 superconducting-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)].