November 16, 1007 (H13) In a recent review article published in Low Temperature Physics [1], G. Wendin and V.S. Shumeiko (Chalmers University of Technology) outline breakthroughs in the observations of macroscopic quantum coherence effects in Josephson circuits, and the demonstrations of the first examples of superconducting qubits.  These pioneering experiments have opened an entirely new (post-classical) dimension for research and applications in the well established field of Josephson electronics. Non-linear electrodynamics of Josephson junction circuits allow for selective manipulation of the quantized energy levels of the circuits, giving the possibility to produce highly non-trivial macroscopic quantum states. Such states can be used for quantum information processing and for applications to microwave quantum optics. In the article the authors introduce basic Josephson circuits for qubit applications (based on SQUIDs and single Cooper-pair boxes), and discuss methods for quantum description of these circuits. Principles of manipulation and readout of superconducting qubits, as well as circuit solutions for qubit-qubit couplings are reviewed and illustrated by recent experiments on different types (phase, flux, charge) of superconducting qubits. Very recently there has been impressive progress with, among other things, the first demonstration of a universal CNOT gate in a superconducting two-qubit circuit [2]. During the next five years one can expect to see functional circuits with 3-5 qubits performing some elementary toy algorithms as well as some first steps toward hybrid processing units through integration of classical control circuits and quantum circuits.

[1] G. Wendin and V.S. Shumeiko, Low Temp. Phys. 33, 724-444 (2007).
[2] J.H.Plantenberg et al., Nature 447, 836-839 (2007).

Fig. 1. Examples of flux qubit circuits developed at TU Delft. Left: flux qubit (three-junction rf SQUID) directly coupled to a readout dc SQUID. Right: two inductively coupled flux qubits with individual readout dc SQUID, used for demonstrating the first general CNOT gate in a superconducting circuit [2].