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(a) The top chip is self-aligned a distance above the bottom chip by use of sapphire spheres of diameter . The top chip pocket diameter is determined by Eq. (1) with a fixed bottom chip pocket depth of . (b) The top chip and bottom chip are separated, showing the alignment sites and the scale of each chip. Note that the top chip is smaller than the bottom chip to allow space for wire bonding. (c) The assembled flip-chip.
(a) Flip-chip circuit drawing shows the simple qubit circuit and the three inductively coupled control coils for microwave excitation, dc flux bias, and dc SQUID bias, as well as the dc three-junction SQUID for qubit readout. (b) A photograph of qubit loop near the final steps of fabrication as patterned on the top chip. A final wiring layer connects the junction and the via (not shown). (c) Photograph of measurement and excitation circuitry as fabricated on the bottom chip. The dashed large rectangle indicates where the qubit will align.
Qubit steps for two different qubit chips (same readout chip) showing different coupling. (a) gap size. The steps are curved due to the large overlap coupling to the dc SQUID. A flux quantum in the qubit is observed with the applied voltage . (b) gap size has weaker coupling and samples just the linear regime of the SQUID. A larger applied voltage is needed to excite a flux quantum with .
Data collected from gap sized flip-chip. (a) Spectroscopy data showing the tunability of the qubit resonant frequency as a function of the applied flux from the bottom chip. The inset shows a zoom in of one of many splittings due to coupling with parasitic two level systems in this qubit. (b) Rabi oscillations in the qubit from microwave excitation. (c) Relaxation time measurement.
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