Nanoscale superconducting quantum bits

Various physical systems were proposed for quantum information processing. Among those nanoscale devices appear most promising for integration in elec tronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. Thes e systems combine intrinsic coherence of the superconducting state with con trol possibilities of single-charge circuits. In the regime where the typic al charging energy dominates over the Josephson coupling, the low-temperatu re dynamics is limited to two states differing by a Cooper-pair charge on a superconducting island. In the opposite regime of prevailing Josephson ene rgy, the phase (or flux) degree of freedom can be used to store and process quantum information. Under suitable conditions the system reduces to two s tates with different flux configurations. Several qubits can be joined toge ther into a register. The quantum state of a qubit register can be manipula ted by voltage and magnetic field pulses. The qubits are inevitably coupled to the environment. However, estimates of the phase coherence time show th at many elementary quantum logic operations can be performed before the pha se coherence is lost. In addition to manipulations, the final state of the qubits has to be read out. This quantum measurement process can be accompli shed using a single-electron transistor for charge Josephson qubits, and a d.c.-SQUID for flux qubits. Recent successful experiments with superconduct ing qubits demonstrate for the first time quantum coherence in macroscopic systems. (C) 2001 Elsevier Science B.V. All rights reserved.