Junjie Li
High-fidelity single-shot readout of superconducting qubits is essential for fault-tolerant quantum computation. Without a quantum-limited amplifier such as a Josephson parametric amplifier, the cryogenic high-electron-mobility-transistor (HEMT) low-noise amplifier (LNA) at the 4 K stage is the dominant noise source in the readout chain. HEMT LNAs are conventionally characterized by Y-factor measurements of the noise temperature $T_N$, independently of the quantum measurement they ultimately serve. Here we present a qubit-in-the-loop benchmarking method. The method uses the two-dimensional IQ histogram of single-shot readout as a direct figure of merit. We apply it to three state-of-the-art cryogenic InP HEMT LNAs with 53\%, 60\%, and 70\% channel indium content, together with a commercial reference amplifier. The 60\% and 70\% devices have similar $T_N$ in Y-factor measurements, both lower than the 53\% device. The 70\% device has the highest gain, but qubit readout SNR and assignment fidelity $\mathcal{F}_a$ instead peak at 60\%. Using this method, we map $\mathcal{F}_a$ against HEMT LNA dc power consumption for each device. $\mathcal{F}_a$ does not simply saturate with increasing power. For several devices it declines above a device-dependent power well below typical operating points. For the 60\% device, which has the highest overall $\mathcal{F}_a$, about 1~mW is needed to maintain $\mathcal{F}_a > 85\%$. Across all four devices, $\mathcal{F}_a > 80\%$ is reached with as little as about 0.3~mW, roughly an order of magnitude below typical operating points. These results establish a qubit-referenced benchmarking methodology and provide practical guidance for amplifier selection and bias in power-constrained multi-qubit readout systems.