cryogens

I’m a low-temperature experimental physicist. You might wonder how us scientists attain the really cold temperatures at which we perform our fancy physics experiments, and I’m here to tell you (I’ll leave the question as to why for another entry!). The first step is often immersion in a very cold liquid, with those maintaining their liquid form waaaaay below the freezing point of water (ice cold!) known as cryogens. You’re probably familiar with these from the movie Terminator 2, in which a tanker of liquid nitrogen (LN2, for those in the know) splits open and freezes the liquid-y metallic T-1000 long enough for Arnold Schwarzenegger to shatter him with a shotgun blast. But of course there are others! The main engines of NASA’s recently defunct Space Shuttle program used a combination of liquid oxygen and liquid hydrogen to produce the explosive force that (along with the solid boosters) propelled Americans into space for the past three decades. But these cryogens have a drawback if you wanna go all the way… (to absolute zero! as we physicists do). They all will eventually freeze.

Frozen things don’t do a good job of low-temperature cooling for a number of reasons, but there is one element that never freezes (unless you put it under enormous pressure, technically!). And that element is helium! Noble helium, chemically inert and easily dismissed as that element that makes your voice squeaky, has many more uses than just filling the party balloons. Actually, the cooling of superconductors used in MRI medical imaging is the major commercial application of helium. Checking in at 4.2 degrees on the Kelvin temperature scale, where 0 means zero, liquid helium is slightly above the temperature where thermal energy ceases to exist. Terrestrial scales of temperature put that at -273.15C or -459.67F. Because of something called quantum zero-point motion, helium remains a liquid from 4.2K all the way down to absolute zero. Simple refrigeration techniques can take you from 4.2K down to around 1K. It takes refrigerators that use even fancier quantum properties of helium to attain temperatures of a hundredth of a degree above absolute zero. And that’s where I spend my time!

In future entries, I will expound upon how cryogens, mainly liquid helium, are used to cool our cool quantum devices, which include superconductors a lot of the time. And why do we care to bring things down to such a level? How do we get liquid helium? What kinds of things are we cooling with it and why? (Hint: they’re all crazy physics materials/devices with potential technological applications.) And perhaps most importantly, why is any of this important?