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We present a new configuration concept in which two similar Josephson junctions are coupled through a capacitor placed in parallel to a dc-superconducting quantum interference device (SQUID) to improve the characteristics of phase qubits. In real coupled quantum systems, because of mutual effects such as crosstalk, entangled quantum states cannot be independently measured. The proposed two-qubit system is demonstrated to have a negligible crosstalk, obtained from the application of a single measurement pulse and an appropriate external flux to one of the junctions and the dc-SQUID, respectively. Surprisingly, the theoretically predicted fidelity for a single-qubit design increases to 99.99% for typical Josephson junction parameters. The high-fidelity measurement is reached even with pulse times less than 1 ns, while the highest fidelities reported so far are obtained by wide temporal pulses through single-shot current pulse measurements. This is accomplished by proper coupling capacitance, which determines the coupling time, the measurement time, and their proportion. Furthermore, the ratio of decoherence time for this structure with respect to the one belonging to a typical qubit is found to be more than 1.42.
Date of Publication: Aug. 2012