Regensburg 2000 – scientific programme
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TT: Tiefe Temperaturen
TT 6: Josephson-Kontakte und SQUIDs I
TT 6.1: Invited Talk
Monday, March 27, 2000, 16:00–16:30, H19
Charge and flux Josephson quantum bits — •Gerd Schön1,2, Yuriy Makhlin1,3, and Alexander Shnirman1 — 1Institut für Theoretische Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe — 2Forschungszentrum Karlsruhe, Institut für Nanotechnologie, D-76021 Karlsruhe — 3Landau Institute for Theoretical Physics, Moscow
Nano-electronic realizations of quantum bits appear most promising for integration in electronic circuits and large-scale applications. In Josephson junctions, where the coherence of Cooper-pair tunneling can be exploited, charge or phase degrees of freedom can be used to process quantum information. Josephson circuits can serve as qubits, with logic states differing by the value of the charge on an island or of the magnetic flux in a loop. We discuss designs with controlled interqubit couplings. Elementary logic gates can be performed by voltage and current pulses. The coherence time is long enough to allow a series of elementary steps.
Information about the quantum state of a charge qubit can be read out by a single-electron transistor. To describe this quantum measurement process we study the evolution of the density matrix of the coupled system. The phase coherence between the logic states of the qubit is lost after a short dephasing time, while their occupations change slowly on a much longer mixing time scale. At intermediate times the probability distribution of measured values of the current in the transistor develops two peaks. This allows to deduce information about the state of the qubit. Shot- and telegraph-noise features show up in the low-frequency current-noise spectrum, thus revealing the relevant time scales.