Prof. Michel H. Devoret

Department of Applied Physics , Yale University
New Heaven, CT

Qubit integrated circuits based on superconducting microwave resonators and tunnel junctions

We have performed new experiments on a superconducting tunnel junction circuit, the "Quantronium"[1], which behaves like a two-level atom. The circuit consists of a Cooper pair transistor, whose degree of freedom is analogous to the position of the electron, in parallel with a large Josephson junction, whose degree of freedom is analogous to the position of the atom. Quantum operations are performed by microwave voltage pulses applied to the gate of the Cooper-pair transistor. The qubit is readout by another microwave pulse which excites the large junction [2]. The phase of the scattered microwave signal carries away the information on the state of the qubit. All manipulations of the qubit, including coupling to another qubit nearby or a microwave quantum bus, can take place at a working point where the transition energy is independent, to first order, of fluctuations in bias parameters. Quantum coherence quality factors in excess of 50,000 are measured by performing a Ramsey fringe experiment. The contrast and visibility of Rabi oscillations can be analyzed in experiments involving several readout pulses in succession. The predicted Quantum Non-Demolition character of the readout scheme is observed partially.

[1] D. Vion, A. Aassime, A. Cottet, P. Joyez, H. Pothier, C. Urbina, D. Esteve and M. H. Devoret, Science 296 (2002) 286.
[2] I. Siddiqi, R. Vijay, F. Pierre, C.M. Wilson, M. Metcalfe, C. Rigetti, L. Frunzio, and M.H. Devoret, Phys. Rev. Lett. 93 (2004) 207002.