Helium-3 Cryostats Versus Dilution Refrigerators
A simple helium-3 evaporation cryostat reaches a few hundred millikelvin, while a dilution refrigerator uses the isotope mixture to go an order of magnitude colder.
Two ways to use helium-3
Helium-3 serves cryogenics in two distinct architectures. A helium-3 cryostat cools by evaporating pure liquid helium-3 under a pump, reaching about 0.3 kelvin. A dilution refrigerator uses a helium-3 / helium-4 mixture and reaches below 0.01 kelvin. Both depend on helium-3, but they exploit different physics and reach very different temperatures.
The evaporation cryostat
In a single-shot helium-3 cryostat, a charge of helium-3 is condensed into a pot and then pumped, often with an adsorption pump of activated charcoal cooled to soak up the vapor. Evaporation cools the remaining liquid to around 0.3 kelvin. When the liquid is exhausted, the run ends and the charge must be recondensed. It is simple, compact, and needs no helium-4 mixture, but it is limited to a few hundred millikelvin and to finite hold times unless made continuous.
Why dilution goes colder
Ordinary evaporation cooling loses effectiveness as temperature drops, because vapor pressure falls exponentially and there is little vapor left to pump. The dilution refrigerator sidesteps this: the effective evaporation is helium-3 crossing into the dilute phase, and because the dilute phase retains helium-3 down to absolute zero, cooling continues far below where plain evaporation stalls. This is the key reason the mixture reaches tens of millikelvin while pure helium-3 evaporation stops near 0.3 kelvin.
Choosing between them
For experiments needing only a few hundred millikelvin, a helium-3 cryostat is simpler and uses less of the scarce isotope. For quantum processors and physics requiring tens of millikelvin, the dilution refrigerator is necessary despite its greater complexity and larger helium-3 inventory. The choice trades temperature reach against simplicity and isotope use.
- Helium-3 evaporation cryostat reaches about 0.3 K
- Dilution refrigerator reaches below 0.01 K
- Evaporation stalls as vapor pressure falls; dilution does not
- Choice trades temperature against simplicity and isotope use