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De Groot, P.C. (author), Van Loo, A.F. (author), Lisenfeld, J. (author), Schouten, R.N. (author), Lupa?cu, A. (author), Harmans, C.J.P.M. (author), Mooij, J.E. (author)We present experimental results on the crosstalk between two ac-operated dispersive bifurcation detectors, implemented in a circuit for high-fidelity readout of two strongly coupled flux qubits. Both phase-dependent and phase-independent contributions to the crosstalk are analyzed. For proper tuning of the phase the measured crosstalk is 0.1%...journal article 2010
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Lupa?cu, A. (author), Bertet, P. (author), Driessen, E.F.C. (author), Harmans, C.J.P.M. (author), Mooij, J.E. (author)We observed the dynamics of a superconducting flux qubit coupled to an extrinsic quantum system (EQS). The presence of the EQS is revealed by an anticrossing in the spectroscopy of the qubit. The excitation of a two-photon transition to the third excited state of the qubit-EQS system allows us to extract detailed information about the energy...journal article 2009
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Picot, T. (author), Lupa?cu, A. (author), Saito, S. (author), Harmans, C.J.P.M. (author), Mooij, J.E. (author)We analyze the relaxation of a superconducting flux qubit during measurement. The qubit state is measured with a nonlinear oscillator driven across the threshold of bifurcation, acting as a switching dispersive detector. This readout scheme is of quantum nondemolition type. Two successive readouts are used to analyze the evolution of the qubit...journal article 2008