And if you need colder than 2mk, nuclear demagnetization: https://en.wikipedia.org/wiki/Magnetic_refrigeration#Nuclear_demagnetization https://en.wikipedia.org

And if you need colder than 2mk, nuclear demagnetization: https://en.wikipedia.org/wiki/Magnetic_refrigeration#Nuclear_demagnetization https://en.wikipedia.org/wiki/Magnetic_refrigeration#Nuclear...