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Comment by ethbro | Hacker News Reader

Would be fascinated to hear anything you (or anyone in a related field) has to say on the challenges for cooling to such low temperatures.

Is it essentially just heat-removal + lots of layers of isolated, effective insulation, or are there fundamental differences when you're going that low?

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I work in this exact field. Yes. It's essentially a thermos. There's several cans nested within each other. Each can hangs from a different refrigeration stage. Typically you go from 300 K (the outer vacuum can), to 77 K (first pulse tube stage), to 4K (second pulse tube stage), then to 300 mK (still of dilution unit), then to base at 10 mK, though the 10 mK stage doesn't have it's own can. Everything is copper or gold plated copper where you want heat to flow, or G-10 (fiberglass) or stainless steel where you don't. There's mechanisms for thermally shorting stages together... sometimes it's a mechanical gripper that pinches an extension from a colder stage, other times it's stainless steel tubing that gets filled with gas (the heat conducting material) when you want to transfer heat, and then evacuated when you don't.

I'm not sure about what's used in all industry applications, but this video explains one implementation of a super-cooling setup: https://www.youtube.com/watch?v=7jT5rbE69ho.

I won't even try to summarise it - I'm sure I'd get it wrong! Fascinating stuff though.

I worked with NMR about 8 years ago in the uni. We used regular liquid helium to get to about 4.2 degrees K. The next step is He 3 isotope which is an order of magnitude more expensive but gets you an order of magnitude cooler (in degrees K). That usually runs in a closed system (heated He is not vented into the atmosphere) because of its cost.

The way cooking it was explained to me is that you try to slow down atoms with lasers, but I don’t know any of the details.

From memory: you fire a laser at a frequency that is only absorbed if relatvistically shifted the way an electron that goes towards the laser would observe. This, the laser delivers momentum only to atoms coming towards it, and is invisible to those at rest or going away. Do from multiple directions and you’ve reduced absolute momentum in every direction.

Claude Cohen danucci received a nobel and a wolf for this (and other things)

https://en.wikipedia.org/wiki/Doppler_cooling

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