Well obviously you can go lower, but 4K is the boiling point of Liquid Helium, so it's the easiest temperature to hold it at.
In the article they mention they chip itself is kept at millkelvin temperatures.
And I disagree about using Fahrenheit - this is not a paper in a journal, it's a lay article. By all means use Fahrenheit if that's what your audience understands. Don't let pedantry get in the way of clarity. (Kelvin has some real meaning, but C vs F is totally arbitrary. One is the freezing point of water, the other is the lowest temperature you can get with a salt/ice mixture.)
In the article they mention they chip itself is kept at millkelvin temperatures.
And I disagree about using Fahrenheit - this is not a paper in a journal, it's a lay article. By all means use Fahrenheit if that's what your audience understands. Don't let pedantry get in the way of clarity. (Kelvin has some real meaning, but C vs F is totally arbitrary. One is the freezing point of water, the other is the lowest temperature you can get with a salt/ice mixture.)