Chapter 18. Josephson-Junction Qubits

  1. Prof. Dr. Samuel L. Braunstein4,
  2. Dr. Hoi-Kwong Lo5 and
  3. Pieter Kok Assistant Editor4
  1. Yuriy Makhlin1,2,
  2. Gerd Schön1,3 and
  3. Alexander Shnirman1

Published Online: 28 JAN 2005

DOI: 10.1002/3527603182.ch18

Scalable Quantum Computers: Paving the Way to Realization

Scalable Quantum Computers: Paving the Way to Realization

How to Cite

Makhlin, Y., Schön, G. and Shnirman, A. (2000) Josephson-Junction Qubits, in Scalable Quantum Computers: Paving the Way to Realization (eds S. L. Braunstein, H.-K. Lo and P. Kok), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, FRG. doi: 10.1002/3527603182.ch18

Editor Information

  1. 4

    University of Wales, Bangor, UK

  2. 5

    MagiQ Technologies, Inc., New York, USA

Author Information

  1. 1

    Institut für Theoretische Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe, Germany

  2. 2

    Landau Institute for Theoretical Physics, Kosygin St. 2, 117940 Moscow, Russia

  3. 3

    Forschungszentrum Karlsruhe, Institut für Nanotechnologie, D-76021 Karlsruhe, Germany

Publication History

  1. Published Online: 28 JAN 2005
  2. Published Print: 20 DEC 2000

ISBN Information

Print ISBN: 9783527403219

Online ISBN: 9783527603183



  • quantum computation;
  • quantum computing;
  • Josephson-junction qubits;
  • Josephson coupling;
  • single-electron transistor (SET)


Among the potential physical implementations of quantum bits (qubits) solid state devices appear most promising for large scale applications and integration in electronic circuits. Devices based on low-capacitance Josephson junctions exploit the coherence of the superconducting state, combined with the possibility to control individual charges by Coulomb blockade effects. The logical states in these systems differ by one Cooper-pair charge. Single- and two-bit operations can be performed by applying a sequence of gate voltages. The phase coherence time is sufficiently long to allow a series of these steps. If, in addition, the strength of the Josephson coupling is controlled (e.g. by currents in SQUID loops) the system is ideal for quantum computation. Recent experiments by Nakamura et al. show the feasibility of this approach.

In addition to the manipulations of qubits the resulting quantum state has to be read out. This quantum measurement process can be accomplished by coupling a single-electron transistor (SET) capacitively to the qubit. We investigate the process by analyzing the time-evolution of the density matrix of the coupled system. The measurement process is characterized by three time scales: the dephasing time, which is fast when a dissipative current flows in the SET, a longer ‘measurement time’ needed to read out the signal, and a third, even longer ‘mixing time’, characterizing measurement induced transitions which destroy the information about the initial quantum state. The noise spectrum of the current in the SET displays these time scales and allows their direct observation.