Liquid Nitrogen with pumping: 40 K for a few thousand dollars.
Run of the mill Liquid Helium: 4 K for tens to hundreds of thousands of dollars.
But for these devices you need 15mK which is reachable only if you mix two different isotopes of Helium and pump the mixture into vacuum. Such a device is up to 1M$ and more.
And the insides of that device are in vacuum (actually, air freezing into ice on top of the chip can be a problem). The brass is basically the heat conductor between the chip and the cold side of your pumped He mixture (which is *not* just sloshing inside the whole body of the cryostat where the chips are).
Another reason you do not want the He sloshing around is because you will be opening this to make changes to the device and do not want all the extremely expensive He3 (the special isotope you need for the mixture) to be lost.
Dilution refrigerator. They are the type of refrigerator used to chill quantum computing devices. The wikipedia article has a pretty good description of how they work. It took me a few reads to understand it!
Liquid Nitrogen with pumping: 40 K for a few thousand dollars.
Run of the mill Liquid Helium: 4 K for tens to hundreds of thousands of dollars.
But for these devices you need 15mK which is reachable only if you mix two different isotopes of Helium and pump the mixture into vacuum. Such a device is up to 1M$ and more.
And the insides of that device are in vacuum (actually, air freezing into ice on top of the chip can be a problem). The brass is basically the heat conductor between the chip and the cold side of your pumped He mixture (which is *not* just sloshing inside the whole body of the cryostat where the chips are).
Another reason you do not want the He sloshing around is because you will be opening this to make changes to the device and do not want all the extremely expensive He3 (the special isotope you need for the mixture) to be lost.