Phase coherent dynamics of a superconducting flux qubit with capacitive bias readout

F. Deppe, M. Mariantoni, E. P. Menzel, S. Saito, K. Kakuyanagi, H. Tanaka, T. Meno, K. Semba, H. Takayanagi, and R. Gross
Phys. Rev. B 76, 214503 – Published 4 December 2007

Abstract

We present a systematic study of the phase coherent dynamics of a superconducting three-Josephson-junction flux qubit. The qubit state is detected with the integrated-pulse method, which is a variant of the pulsed switching-dc-superconducting quantum interference device (SQUID) method. In this scheme, the dc SQUID bias current pulse is applied via a capacitor instead of a resistor, giving rise to a narrow bandpass instead of a pure low-pass filter configuration of the electromagnetic environment. Measuring one and the same qubit with both setups allows a direct comparison. With the capacitive method about four times faster switching pulses and an increased visibility are achieved. Furthermore, the deliberate engineering of the electromagnetic environment, which minimizes the noise due to the bias circuit, is facilitated. Right at the degeneracy point, the qubit coherence is limited by energy relaxation. We find two main noise contributions. White noise limits the energy relaxation and contributes to the dephasing far from the degeneracy point. 1f noise is the dominant source of dephasing in the direct vicinity of the optimal point. The influence of 1f noise is also supported by nonrandom beatings in the Ramsey and spin echo decay traces. Numeric simulations of a coupled qubit-oscillator system indicate that these beatings are due to the resonant interaction of the qubit with at least one pointlike fluctuator, coupled especially strongly to the qubit.

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  • Received 17 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.214503

©2007 American Physical Society

Authors & Affiliations

F. Deppe1,2,3, M. Mariantoni1, E. P. Menzel1, S. Saito2,3, K. Kakuyanagi2,3, H. Tanaka2,3, T. Meno4, K. Semba2,3, H. Takayanagi3,5, and R. Gross1

  • 1Walther-Meißner-Institut, Walther-Meißner-Straße 8, D-85748 Garching, Germany
  • 2NTT Basic Research Laboratories, NTT Corporation, Kanagawa 243-0198, Japan
  • 3CREST, Japan Science and Technology Agency, Saitama 332-0012, Japan
  • 4NTT Advanced Technology, NTT Corporation, Kanagawa 243-0198, Japan
  • 5Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

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Issue

Vol. 76, Iss. 21 — 1 December 2007

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