Decoupling a Cooper-pair box to enhance the lifetime to 0.2 ms

Abstract

We present a circuit QED experiment in which a separate transmission line is used to address a quasi-lumped element superconducting microwave resonator which is in turn coupled to an Al/AlO${}_x$/Al Cooper-pair box (CPB) charge qubit. In our measurements we find a strong correlation between the measured lifetime of the CPB and the coupling between the qubit and the transmission line. By monitoring perturbations of the resonator's 5.44 GHz resonant frequency, we have measured the spectrum, lifetime ($T_1$), Rabi, and Ramsey oscillations of the CPB at the charge degeneracy point while the CPB was detuned by up to 2.5 GHz . We find a maximum lifetime of the CPB was $T_1=200\mu$s for $f=4$ to 4.5 GHz. Our measured $T_1$'s are consistent with loss due to coupling to the transmission line, spurious microwave circuit resonances, and a background decay rate on the order of $5\times 10^3$ s${}^{-1}$ of unknown origin, implying that the loss tangent in the AlO${}_x$ junction barrier must be less than about $4\times 10^{-8}$ at 4.5 GHz, about 4 orders of magnitude less than reported in larger area Al/AlO${}_x$/Al tunnel junctions.

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