cryogenic attenuation
Imagine running a microwave cable from a warm room down into a fridge that holds your qubit at a hair above absolute zero. The cable does its job, but it also smuggles down something unwanted: faint thermal noise from the warm end, plus heat. Even a tiny trickle of room-temperature noise photons would scramble a delicate qubit. Cryogenic attenuation is the fix: small attenuators bolted onto the cable at each cold plate inside the fridge, deliberately weakening the signal so the noise riding along with it gets thrown away too.
An attenuator is just a resistor network that absorbs most of the power passing through and re-emits only the gentle thermal noise of its own temperature. So an attenuator sitting on the 4-kelvin plate replaces warm noise with 4-kelvin noise; another on the coldest plate replaces that with millikelvin noise. By staging attenuation down the fridge, each stage hands the next a cleaner signal, and the absorbed heat is dumped into a plate that the fridge can actually cool. Your control pulse arrives weak but bathed in a very cold, quiet environment.
The catch is that you are throwing away signal on purpose, so the room-temperature electronics must send much stronger pulses to compensate, and the absorbed power adds to the fridge's limited cooling budget. Designers balance how much to attenuate at each stage against how much heat each stage can take. It works well today and is standard practice, but it is also part of why scaling to many qubits is hard: every extra control line needs its own attenuated cable, and the heat and clutter add up fast.
An attenuator with power loss G (e.g. G = 0.01 for 20 dB) divides the incoming noise N_in by G, but adds back N_atten set by its own physical temperature — so a colder attenuator hands on a quieter signal.
A common starting point is roughly 20 dB on the 4 K stage and another 20 dB near the qubit, but the exact split is tuned per fridge and per line; there is no single right answer.