"Manipulating the quantum state of an electrical circuit"
Michel Devoret
Yale University

Abstract

We have designed and operated a novel superconducting tunnel junction circuit [1] that behaves like a two-level quantum system (qubit). The circuit consists of a single Cooper pair transistor in parallel with a large current-biased Josephson junction.  One-qubit operations are performed by microwave voltage pulses applied to the gate of the Cooper-pair transistor.  The qubit is readout by a radio-frequency current pulse applied to the large junction. Depending on the state of the qubit, this junction switches or not to the voltage state. All manipulations of the qubit can take place at a working point where the transition energy is independent of small fluctuations in bias parameters. By performing a Ramsey fringe experiment at this optimum bias point, we have measured a quantum coherence quality factor Q_phi=25000. Intensity of the dephasing noise was extracted in a separate experiment from the broadening of the transition lineshape as a function of the deviation from this bias point.

[1] D. Vion, A. Aassime, A. Cottet, P. Joyez, H. Pothier, C. Urbina, D. Esteve and M.D.; Science 296 (2002) 286