Quantum computing based on space states without charge transfer

An implementation of a quantum computer based on space states in double quantum dots is discussed. There is no charge transfer in qubits during calculation, therefore, uncontrollable entan-glement between them due to long-range Coulomb interaction is suppressed. Other plausible sources of decoherence caused by interaction with phonons and gates could be substantially suppressed in the structure too. We also demonstrate how all necessary quantum logic operations, initialization, writing, and read-out could be carried out in the computer.Comment: 7 pages, 4 figures, RevTeX forma

Superconducting single-mode contact as a microwave-activated quantum interferometer

The dynamics of a superconducting quantum point contact biased at subgap voltages is shown to be strongly affected by a microwave electromagnetic field. Interference among a sequence of temporally localized, microwave-induced Landau-Zener transitions between current carrying Andreev levels results in energy absorption and in an increase of the subgap current by several orders of magnitude. The contact is an interferometer in the sense that the current is an oscillatory function of the inverse bias voltage. Possible applications to Andreev-level spectroscopy and microwave detection are discussed

Resonant microwave properties of a voltage-biased single-Cooper-pair transistor

We consider the microwave dynamics and transport properties of a voltage-biased single-Cooper-pair transistor. The dynamics is shown to be strongly affected by interference between multiple microwave-induced inter-level transitions. As a result the magnitude and direction of the dc Josephson current are extremely sensitive to small variations of the bias voltage and to changes in the frequency of the microwave field.Comment: 6 pages, 4 figure