Figure 1. Bistability and drift of the qubit transition frequency.

(a) Ramsey fringe experiment (dots) and best-fit sum of two decaying sinusoids (curve). The reference oscillator is detuned 1 MHz from the average qubit transition frequency ω₀₁/2π=4.387 GHz. (b) Sketch of the charge dispersion of the two lowest-energy levels of the transmon qubit, showing 2e periodicity. QP tunnelling across the junction shifts n_g by ±e, resulting in two transition frequencies f_e and f_o (not to scale). (c) Repeated Ramsey experiments (15 s each) show a symmetric drift of f_e and f_o around ω₀₁/2π, arising from background charge motion. The frequency difference 2Δf=f_e−f_o ranges from 0 to 1.76 MHz (see also Supplementary Fig. S1).