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SYQI: Quantum information processing in condensed media

SYQI 2: Quantum information processing in condensed media II

SYQI 2.3: Invited Talk

Monday, March 24, 2003, 16:15–16:45, HSZ/01

Superconducting devices for quantum information processing — •Yuriy Makhlin, Gerd Schön, and Alexander Shnirman — Institut für Theoretische Physik, Universität Karlsruhe, 76128 Karlsruhe

We report on recent progress in theory and experiment of Josephson-junction quantum information circuits. Coherent oscillations of coupled qubits were observed, and in single qubits the coherence time reached 1 µs. We discuss various designs of single and coupled qubits and their sensitivity to external noise.

A related issue is the efficiency of various mesoscopic detectors of the quantum state. Important aspects, such as the proximity to a strong quantum-limited or a QND measurement, can be expressed via the hierarchy of time scales during the readout process, namely the dephasing, measurement (the output signal exceeds the noise), and mixing (relaxation) times.

In order to describe noise effects and the measurment process we developed a systematic description, which also accounts for higher-order contributions to dephasing. They become relevant: (i) when the linear coupling to the environment is suppressed, as in recently proposed designs of Josephson qubits and quantum detectors; (ii) for transverse coupling to a source of 1/f noise. Our results demonstrate the importance of non-Gaussian noise for the qubit dynamics.