Measurements of Capacitive Coupling Within a Quadruple-Quantum-Dot Array

We present measurements of the capacitive coupling energy and the interdot capacitances in a linear quadruple-quantum-dot array in undoped Si/SiGe. With the device tuned to a regime of strong (>1 GHz) intra-double-dot tunnel coupling, as is typical for double-dot qubits, we measure a capacitive coupling energy of 20.9 +/- 0.3 GHz. In this regime, we demonstrate a fitting procedure to extract all the parameters in the four-dimensional Hamiltonian for two capacitively coupled charge qubits from a two-dimensional slice through the quadruple-dot charge-stability diagram. We also investigate the tunability of the capacitive coupling energy, using interdot barrier gate voltages to tune the inter- and intra-double-dot capacitances, and change the capacitive coupling energy of the double dots over a range of 15-32 GHz. We provide a model for the capacitive coupling energy based on the electrostatics of a network of charge nodes joined by capacitors, which shows how the coupling energy should depend on inter-double-dot and intra-double-dot capacitances in the network, and find that the expected trends agree well with the measurements of coupling energy.