Low-Power HEMT LNAs for Quantum Computing

Licentiate thesis, 2023

The rapid development of quantum computing technology predicts much more qubits to handle in the detection, readout, and amplification of qubits than in today's system. Due to the limited cooling capability of the dilution refrigerator, the current low-noise amplifiers (LNAs) are in need of ten to hundred times reduced dc power consumption yet with lowest noise temperature at qubit readout frequencies, typcially 4-12 GHz. Cryogenic indium phosphide (InP) high electron mobility transistor (HEMT) LNAs, are the standard qubit amplifier at 4 K in today's superconducting quantum system. However, the power consumption of current InP HEMT LNAs is still too high for future quantum system up-scaling.

A small-signal noise model of a 100-nm gate-length InP HEMTs has been characterized and extracted at 4 K ambient under low-power bias down to 1 μW. The extracted low-power small-signal noise models revealed fast degradation points of drain voltage bias for RF and noise performance.

The design goals of the cryogenic LNA were tailored for a superconducting qubit readout application based on the extracted low-power small-signal noise model of the InP HEMT for optimum noise and power consumption trade-off. A cryogenic InP HEMT hybrid LNA operating in the 4-6 GHz frequency range at 200 μW with an average noise temperature of 2.0 K has been designed, fabricated, and successfully demonstrated, validating the extracted model and design methodology.

An epitaxially-optimized InP HEMT was modeled with the low-power methodology. The comparison of the small-signal noise model parameters to the standard InP HEMT showed improved transconductance, matching, and noise at the same bias power. The demonstrated three-stage cryogenic 4-6 GHz LNA equipped with an optimized HEMT as the first stage achieved 2.0 K average noise temperature at 100 μW dc power dissipation, representing a new state-of-the-art. This licentiate thesis has presented experimental evidence that there is large potential in reducing dc power in the cryogenic InP HEMT LNA for qubit readout which can be important for the planned up-scaling in future quantum computing.