Sensitive radio-frequency measurements of a quantum dot by tuning to perfect impedance matching

TL;DR

This paper demonstrates perfect impedance matching in radio-frequency measurements of a quantum dot, significantly improving sensitivity and bandwidth, and benchmarks the approach for potential single-shot qubit readout.

Contribution

The authors realize and demonstrate perfect impedance matching using voltage-tunable varactors, achieving high sensitivity and bandwidth in quantum dot measurements.

Findings

Capacitance sensitivity of 1.6 aF/√Hz achieved

Bandwidth above 15 MHz demonstrated

Proportionality between conductance and capacitance in Coulomb blockade

Abstract

Electrical readout of spin qubits requires fast and sensitive measurements, but these are hindered by poor impedance matching to the device. We demonstrate perfect impedance matching in a radio-frequency readout circuit, realized by incorporating voltage-tunable varactors to cancel out parasitic capacitances. In the optimized setup, a capacitance sensitivity of $1.6~\mathrm{aF}/\sqrt{\mathrm{Hz}}$ is achieved at a maximum source-drain bias of $170~\mu$V root-mean-square and with bandwidth above $15~$MHz. Coulomb blockade is measured via both conductance and capacitance in a quantum dot, and the two contributions are found to be proportional, as expected from a quasistatic tunneling model. We benchmark our results against the requirements for single-shot qubit readout using quantum capacitance, a goal that has so far been elusive.