Model for an irreversible bias current in the superconducting qubit measurement process

G. D. Hutchinson*, C. A. Holmes, T. M. Stace, T. P. Spiller, G. J. Milburn, S. D. Barrett, D. G. Hasko, D. A. Williams

*Corresponding author for this work

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The superconducting charge-phase "quantronium" qubit is considered in order to develop a model for the measurement process used in the experiment of Vion [Science 296, 886 (2002)]. For this model we propose a method for including the bias current in the readout process in a fundamentally irreversible way, which to first order is approximated by the Josephson junction tilted-washboard potential phenomenology. The decohering bias current is introduced in the form of a Lindblad operator and the Wigner function for the current-biased readout Josephson junction is derived and analyzed. During the readout current pulse used in the quantronium experiment we find that the coherence of the qubit initially prepared in a symmetric superposition state is lost at a time of 0.2 ns after the bias current pulse has been applied, a time scale that is much shorter than the experimental readout time. Additionally we look at the effect of Johnson-Nyquist noise with zero mean from the current source during the qubit manipulation and show that the decoherence due to the irreversible bias current description is an order of magnitude smaller than that found through adding noise to the reversible tilted-washboard potential model. Our irreversible bias current model is also applicable to persistent-current- based qubits where the state is measured according to its flux via a small-inductance direct-current superconducting quantum interference device.

Original languageEnglish
Article number062302
JournalPhysical Review A
Issue number6
Publication statusPublished - 11 Dec 2006

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