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 π-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-wave–s-wave Josephson-junction qubits introduced by Ioffe et al. [Nature (London) 398, 679 (1999)].