Fig. 1: rf-driven electron cascade.
a,b, Schematics of the top view (a) and cross section (b) (white dashed line in a) of the QD array. Gates G1 and G2 define the QDs Q1 and Q2, which are tuned to the two-electron occupancy. The DQD is capacitively coupled to dot QME, which is occupied by many electrons and is controlled by gate GS. Arrows indicate single-electron tunnelling events. c, Schematic of the rf resonator bonded to the ohmic contact of the device, including an equivalent circuit representation of the QD array as a spin-dependent variable capacitor \({C}_{{{\rm{Q}}}_{\mathrm{ME}}}(\left|\text{S}\right\rangle )\). The resonator is formed by an off-chip superconducting spiral inductor L = 136 nH arranged in parallel with the parasitic capacitance CP = 0.4 pF of the assembly. Connected to the transmission line Z0 via a coupling capacitor CC = 0.1 pF, rfin(out) represents the incident (reflected) rf signal on the resonator. d, Charge stability diagram of the DQD as a function of gate voltages \({V}_{{{\rm{G}}}_{1}}\) and \({V}_{{{\rm{G}}}_{2}}\). Vrf denotes the demodulated rf voltage. e,f, Schematic representations of the cascade process in which an rf excitation with amplitude Arf synchronously drives charge transitions within the QD array. The reservoir is shown as the shaded region, and the dashed lines mark its Fermi level. e, A change in the charge occupation from (\({N}_{{{\rm{Q}}}_{2}}\), \({N}_{{{\rm{Q}}}_{1}}\), \({N}_{{{\rm{Q}}}_{\mathrm{ME}}}\)) = (1, 1, N) to (0, 2, N − 1) raises the electrochemical potential of the QME above the Fermi level, causing one electron to synchronously escape to the reservoir. f, When the DQD is driven back to (1, 1, N), an electron tunnels back from the reservoir to QME.
