Superconducting Stub Tuner Impedance Matching Circuit for High-Frequency Measurements of Nanoscale Devices

Radio-frequency reflectometry measurements in semiconductor devices such as single-electron transistors and quantum point contacts (QPC) allow time-resolved charge detection in gated quantum dots, one of the leading platforms to host spin qubits. Quantum dots and QPCs are high impedance devices, and one needs to impedance-match these to the characteristic impedance Z0 of the waveguide to carry out any transport measurements beyond a few MHz in frequency. In this work, we use a stub tuner as the impedance-matching circuit for performing high-frequency measurements of a QPC. We have designed a stub tuner to impedance match the QPC at a frequency of 3 GHz for operating it in the middle of the last conductance plateau, where the transconductance shows a maximum variation. Stub tuners of desired lengths (D1 = 9.643 mm and D2 = 10.199 mm) have been fabricated and galvanically coupled to the QPC. The reflection coefficient measured at 4 K shows variation with QPC conductance.