# What cryogenic systems are suitable for superconducting qubits?

Is a dilution refrigerator the only way to cool superconducting qubits down to 10 millikelvin? If not, what other methods are there, and why is dilution refrigeration the primary method?

• Heather, I feel this might become a list question. Perhaps you could narrow it down? At this time, there probably is a small number of methods (maybe just one), but in the future there might be more, which would mean that more answers would be added... – ItamarG3 Mar 12 '18 at 19:59

Is a dilution refrigerator the only way to cool superconducting qubits down to 10 millikelvin?

There's another type of refrigerator that can get to 10 mK: the adiabatic demagnetization refrigerator (ADR).$^{[a]}$

why is dilution refrigeration the primary method?

To understand that, let's talk about one of the main limitations of the ADR.

An ADR usually gets to about 3K with a helium compressor. That compressor can run all the time, so the refrigerator can sit at 3K indefinitely. To get down to mK temperatures, the ADR works like this:

1. Raise the magnetic field surrounding a solid with nuclear spins. This aligns the spins.
2. Slowly turn the field off. This allows the spins to randomize their direction, which absorbs entropy from the surroundings and lowers the temperature.
3. Once the field is back to zero, we've sucked enough heat out of the surroundings to bring them to mK temperatures.

This is all great and it really works, but it's a "one-shot" process. Once the field is down to zero, you can't go any lower. Heat from the surroundings, such as the room temperature outer parts of the refrigerator, leak heat into the part you're trying to keep cold, and since we've already lowered the magnetic field to zero, we can't do anything to remove that heat. Therefore, after cooling the ADR, it starts to warm up (hopefully slowly enough to run your experiment).

It's typical for an ADR to stay below 100mK for maybe twelve hours, although that number depends a lot on how many wires you have running to the cold part of the ADR. After the temperature rises above what you want, you have to raise the magnetic field again and slowly lower it to re-cool. Raising and lowering the field takes a while and heats up the refrigerator, and that big magnetic field is often incompatible with superconducting qubit experiments, so you can't run experiments while you're in that stage of the process.

$[a]$: Apologies for not finding a better link. Edits on that are welcome.