Regensburg 2004 – scientific programme
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TT: Tiefe Temperaturen
TT 29: Quantenkoh
ärenz und Quanteninformationssysteme II
TT 29.5: Talk
Thursday, March 11, 2004, 15:30–15:45, H19
Investigation of a superconducting flux qubit with SQUID readout — •S. Linzen1, B.L.T. Plourde1, T.L. Robertson1, T. Hime1, P.A. Reichardt1, C.E. Wu1, F. Wilhelm2, and John Clarke1 — 1Department of Physics, University of California, Berkeley, USA — 2Sektion Physik, Ludwig-Maximilians-Universität, München, Germany
We have studied large inductance flux qubits based on submicron Al/AlOx/Al tunnel junctions. A dc SQUID is used to distinguish the two fundamental states of the three-junction qubit by measuring the switching probability with a pulsed bias current. We have optimized the qubit flux signal and the coupling to the SQUID to minimize decoherence. Single-shot readout has been reached by on-chip shunting of each of the two SQUID junctions with a resistor and capacitor in series. We can still resolve the qubit state with 60% fidelity with a single measurement without the RC shunts. We measure the qubit energy levels spectroscopically using microwave pulses in the wide frequency range 100 MHz to 20 GHz while adjusting the fluxes in the SQUID and qubit by means of static currents in two inductively coupled lines on-chip. The repetition rate of the readout pulses is limited by hot electron generation in the SQUID tunnel junctions when the SQUID switches out of the supercurrent state. The influence of the repetition rate on the resonance peak widths and the relaxation time will be discussed. We observe long relaxation times, more than 20 µ s, and dephasing times of about 5 ns when the repetition rate is lower than 2 kHz.
This work was supported by Alexander von Humboldt-Foundation, AFOSR, ARO, and ARDA.