How do I select a cryogenic low noise amplifier for qubit readout at 4 to 8 GHz?

A cryogenic low-noise amplifier (LNA) for qubit readout at 4-8 GHz is typically a HEMT (High Electron Mobility Transistor) amplifier mounted at the 4K stage of the dilution refrigerator. It provides the first stage of amplification for the weak qubit readout signal after the quantum-limited amplifier (if present) or directly after the circulator chain. Key selection criteria: noise temperature (2-5 K at 4K physical temperature, equivalent to noise figure of 0.03-0.07 dB), gain (35-40 dB to overcome cable losses from 4K to room temperature), bandwidth (4-8 GHz or wider for multiplexed readout), power dissipation (5-15 mW at the 4K stage, within the 1-2W cooling power budget of the pulse tube), input P1dB (-30 to -25 dBm, sufficient for the amplified readout signal without compression), output P1dB (-5 to +5 dBm for driving the lossy cable to room temperature), and DC power supply requirements (typical: Vd = 0.5-1.2V, Id = 5-15 mA). Leading cryogenic LNA products: Low Noise Factory LNF-LNC4_8C (4-8 GHz, 2K noise temperature, 40 dB gain, 8 mW dissipation), Cosmic Microwave Technology CITLF3 (1-12 GHz, 3K noise, 38 dB gain), and custom university designs from Caltech, JPL, and Chalmers. InP HEMT transistors (Northrop Grumman/HRL NGC100) are the standard active device technology, achieving noise temperatures approaching the quantum limit (hf/2k ≈ 0.1K at 5 GHz) when operated at millikelvin temperatures.