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Superconductor Science and Technology

Pi-phase shift Josephson structures

Hans Hilgenkamp

Published Feb 20, 2008• 42 citations
Publisher

Abstract

One of the unique aspects of superconductivity is the phase coherence of the macroscopic wavefunction. It provides the basis for most concepts of superconducting electronic devices, such as SQUIDs and RSFQ circuitry. The phase coherence also underlies the principle of fluxoid quantization in superconducting ring structures. In recent years, an interest has arisen in introducing phase-shifting elements in superconducting circuits. Here, I will describe various techniques to implement such a phase shift, outline the potential interest for device applications and will attempt to provide an outlook for future developments in this field. Rather than providing an extensive review for a general readership, this paper aims to briefly introduce various aspects of phase-shift devices to experts in the field of Josephson electronics and to list the most important recent experimental developments in this field.

DOI
10.1088/0953-2048/21/3/034011

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One of the unique aspects of superconductivity is the phase coherence of the macroscopic wavefunction. It provides the basis for most concepts of superconducting electronic devices, such as SQUIDs and RSFQ circuitry. The phase coherence also underlies the principle of fluxoid quantization in superconducting ring structures. In recent years, an interest has arisen in introducing phase-shifting elements in superconducting circuits. Here, I will describe various techniques to implement such a phase shift, outline the potential interest for device applications and will attempt to provide an outlook for future developments in this field. Rather than providing an extensive review for a general readership, this paper aims to briefly introduce various aspects of phase-shift devices to experts in the field of Josephson electronics and to list the most important recent experimental developments in this field.
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We review the physical basis of and recent developments in ferromagnetic Josephson junctions with a focus on the π state, which is useful for applications to superconducting devices. Among various novel phenomena that emerge from the interaction between the superconductor and ferromagnet, the π state (junction) works as a phase shifter in superconducting devices, and several proposals and demonstrations of applications to quantum computing elements (qubits), classical superconducting logic circuits, and cryogenic memory have been presented. As a new research trend in superconducting spintronics, we also introduce recent interesting works on the long-range Josephson effect caused by spin-triplet Cooper pairs.
Recent demonstrations of macroscopic quantum coherence in Josephson junction based electronic circuits have opened an entirely new dimension for research and applications in the established field of Josephson electronics. In this article we discuss basic Josephson circuits for qubit applications, methods of quantum description of these circuits, and circuit solutions for qubit couplings. Principles of manipulation and readout of superconducting qubits are reviewed and illustrated with recent experiments using various qubit types.
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 $\ensuremath{\pi}$ junctions, Josephson junctions with a ground state characterized by a \ensuremath{\pi}-phase shift across, have attracted much attention. We show how to make use of such $\ensuremath{\pi}$ 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 $\ensuremath{\pi}$ junction, we show how to construct effective junctions with degenerate minima characterized by phase shifts 0 and $\ensuremath{\pi}$ 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)].