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Description of the qubit and summary of coherence times. (a) The fabrication process involves a thin seed layer of SiN on top of Silicon. (b) Micrograph of the transmon, and zoom-in of the Josephson junction. (c) T 1 versus finger width dimension, keeping the finger width and separation constant for an interdigitated capacitor of total capacitance . Coherence times of qubits using aluminum (circles) for the capacitor structure improve with increasing finger width. The dashed line is a guide to the eye. A dramatic improvement is observed using TiN (squares). All qubit frequencies were in the range of and the multi-mode Purcell limit is predicted to be far away from observed coherence times ( ).
Representative plots for (a) energy relaxation and (b) Ramsey fringes, which give T 1 and , respectively, (sample B) together with a histogram for individually fitted traces run over the course of 18 h.
Frequency shift versus temperature. For the fundamental resonance at 6.57 GHz, the data (open circles) show a much more pronounced positive shift than predicted (dashed line) assuming . A similar behavior is found for the first harmonic at 13.14 GHz (crosses) but the discrepancy compared the prediction (solid line) is not as large. The overall discrepancy is reduced by introducing lower in the model, which would not be consistent with measured qubit coherence times and assuming the resonator is limited by the same TLS loss. Furthermore, for both the fundamental resonance and first harmonic, the predicted local minimum in frequency shift is not observed. Simulations assume and a kinetic inductance fraction of 0.54, which affect the negative frequency trend at elevated temperature. The simulation traces are offset by for clarity.
Measured and extracted qubit parameters. Sample (D) was connected using poorly thermalized feed lines, likely causing reduced times.
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