An attenuator on a drive line reduces the signal power so that thermal noise from warmer stages is also suppressed. You typically place attenuators at multiple stages (20 dB at 4 K, 20 dB at Cold Plate, 20 dB at Mixing Chamber) so the noise floor drops at each step. A HEMT amplifier boosts the tiny readout signal, but it dissipates about 10 milliwatts — far too much heat for the mixing chamber — so it sits at 4 K where the cooling power can handle it. Circulators on the readout line protect the qubit from amplifier back-action noise. Low-pass filters block high-frequency noise on flux and DC lines.

Think of the signal path as a water pipe system. Attenuators are pressure regulators that reduce flow at each stage. The HEMT amplifier is a pump that boosts a weak trickle into a measurable flow — but it generates heat, so it must be in a room with good ventilation (4 K stage). Circulators are one-way valves that prevent the pump's vibrations from traveling back to the delicate equipment (the qubit).

The attenuation cascade on XY drive lines typically totals 60-70 dB distributed across 3-4 stages. Each attenuator dissipates power as heat, so placing them at colder stages requires careful thermal budgeting. HEMT amplifiers provide approximately 30 dB of gain with a noise temperature of 2-5 K. They must be at 4 K: placing them colder would overload the stage's cooling power, while placing them warmer would add unacceptable noise. Circulators provide approximately -0.5 dB forward insertion loss and -20 dB reverse isolation, preventing the HEMT's noise from reaching the qubit. IR and eccosorb filters block infrared radiation that can excite quasiparticles in superconducting circuits.