Fast time-domain current measurement for quantum dot charge sensing using a homemade cryogenic transimpedance amplifier

We developed a high-speed and low-noise time-domain current measurement scheme using a homemade GaAs high-electron-mobility-transistor-based cryogenic transimpedance amplifier (TIA). The scheme is versatile for broad cryogenic current measurements, including semiconductor spin-qubit readout, owing to the TIA's having low input impedance comparable to that of commercial room-temperature TIAs. The TIA has a broad frequency bandwidth and a low noise floor, with a trade-off between them governed by the feedback resistance $R_{FB}$. A lower $R_{FB}$ of 50 k$Ω$ enables high-speed current measurement with a -3dB cutoff frequency $f_{-3dB}$ = 28 MHz and noise-floor $NF = 8.5 \times 10^{-27}$ A$^{2}$/Hz, while a larger $R_{FB}$ of 400 k$Ω$ provides low-noise measurement with $NF = 1.0 \times 10^{-27}$ A$^{2}$/Hz and $f_{-3dB}$ = 4.5 MHz. Time-domain measurement of a 2-nA peak-to-peak square wave, which mimics the output of the standard spin-qubit readout technique via charge sensing, demonstrates a signal-to-noise ratio (SNR) of 12.7, with the time resolution of 48 ns, for $R_{FB}$ = 200 k$Ω$, which compares favorably with the best-reported values for the radio-frequency (RF) reflectometry technique. The time resolution can be further improved at the cost of the SNR (or vice versa) by using an even smaller (larger) $R_{FB}$, with a further reduction in the noise figure possible by limiting the frequency band with a low-pass filter. Our scheme is best suited for readout electronics for cryogenic sensors that require a high time resolution and current sensitivity and thus provides a solution for various fundamental research and industrial applications.