Superconducting technologies have the unique potential for realizing compact solid-state devices with controllable macroscopic quantum properties and long coherence time. They represent probably the most realistic approach for a technology of quantum computers. In superconductors, all electrons are condensed in the same macroscopic quantum state, separated by a gap from the many quasi-particle states. Superconductors can be weakly coupled with Josephson tunnel junctions (regions where only a thin oxide separates them). The current through a Josephson junction depends upon the phase differences between the superconductors which act as non-commuting conjugate quantum variables to the charges of isolated islands. That makes it possible to construct qubits using superconductors (SQUBIT).
So far, superconducting electronics has not been able to compete with Si- and GaAs-technology in the field of computers, not even for special supercomputers. However, in the emerging field of Quantum Computing the situation is completely different. Now ``quantum coherence'' is the key issue and superconductivity has great advantages due to its built-in principle of ``macroscopic quantum coherence''. An important feature of superconducting junctions is a possibility to reach long decoherence times. The main reason for that is a weak sensitivity of properly designed SQBITs to external electric fields produced by charge fluctuations. A very important step has very recently been taken by Nakamura et al. at the NEC research laboratory in Japan [2], who have demonstrated that it is possible to manipulate qubits in a quantum coherent way for reasonably long times in a Cooper pair box.
The role of dissipation and it's influence on decoherence in these circuits needs to be understood and controlled better. Studies of this issue will be one of the directions of our activity along the project. They will include description of the transfer of Cooper pairs in gated arrays of small Josephson junctions in the adiabatic regime, as well as the control of Josephson currents by manipulating Andreev levels in ballistic junctions. The main focus will be made on the interaction of these systems with environment which is the source of decoherence. Another direction is to suggestion and investigation of new principle for superconducting qubits. In particular, a coherent charge transfer by ``shuttling'' of Cooper pairs will be studied.
In this research we will use experience from previous activities on physics of fluctuations in various mesoscopic systems: point contacts [3], SQUIDs [4], Coulomb-blockaded [5,6] and ballistic [7] superconductor devices, and Andreev quantum interferometers [8].