Fig. 2: SQUID array characterization.
a Equivalent lumped electrical circuit of the device composed of N SQUIDs in series. The static flux is controlled via an external coil and a microwave signal can be applied to the flux line (purple) for fast frequency modulation. The resonator is coupled to a feedline in a notch configuration. b, f Optical micrograph of the two SQUID array resonators studied in this work with N = 10 (red) and N = 32 (blue) SQUIDs, with details on the Josephson junction shown in the inset where the white scale bar represents 5 μm. c, d For the Kerr case (N = 10), the magnitude and phase of the transmission coefficient through the feedline as a function of drive power at the sample. The black dashed curve indicates where the phase of S21 is zero according to numerical simulations, highlighting the position of the multiphoton resonances. e Selected traces of the data reported in (d). Experimental data are shown with circle markers whose colors correspond to the ticks in (d). Numerical fits to a full quantum model are shown in black solid lines. The gray dotted vertical lines indicate the position of the multiphoton resonances obtained from numerical simulations. g–i Same measurement as in (c–e), but for the Duffing case. The dashed curves in (g, h) indicate the minima of ∣S21∣ obtained from a full quantum simulation.
