Fig. 1

Sample and simplified circuit diagram. a False color optical micrograph of the device showing the substrate (dark gray), the Al superconducting structures forming the groud plane (light gray), the DQD Au gate leads (yellow), the SQUID array resonator (red), its microwave feedline (green), the single island transmon (orange), its readout 50 Ω coplanar waveguide resonator (blue), and the flux line (purple). b Enlarged view of the sample area enclosed by the blue dashed line in panel (a). c Enlarged view of the coupling side of the SQUID array. d Electron micrograph of the DQD showing its electrostatic top gates (Al-light gray) and the plunger gate coupled to the SQUID array (red). e Electron micrograph of the transmon SQUID. f Circuit diagram schematically displaying the DQD [with its source (S) and drain (D) contact], capacitively coupled to the SQUID array resonator, which in turn is coupled to the transmon. The transmon and the SQUID array are respectively capacitively coupled to a 50 Ω CPW resonator and microwave feedline. Their resonance frequencies can be tuned by using a flux line and a coil schematically shown in the circuit diagram. The color code is consistent with the optical micrographs