The Dilution Refrigerator
A dilution refrigerator reaches millikelvin temperatures by pumping helium-3 across a phase boundary inside a mixture of helium-3 and helium-4.
What it does
A dilution refrigerator is the workhorse cooler for superconducting quantum processors and many low-temperature physics experiments. It produces continuous cooling below about 300 millikelvin and, in practice, reaches a base temperature of roughly 5 to 10 millikelvin. Unlike a one-shot cooler, it runs indefinitely because it recirculates a working fluid in a closed loop.
The cooling comes from the physics of a dilute mixture of two helium isotopes. Below about 0.87 K a liquid mixture of helium-3 and helium-4 separates into two phases: a concentrated phase that is almost pure helium-3, floating on a dilute phase in which helium-3 is dissolved in superfluid helium-4. Forcing helium-3 atoms to cross from the concentrated phase into the dilute phase absorbs heat, much as evaporation cools a liquid, but the effect persists all the way down to absolute zero.
The circulating loop
Helium-3 is the mobile species. It is pumped away from the dilute phase in the still, driven back down through counterflow heat exchangers, and returned to the mixing chamber where it crosses the phase boundary and provides cooling. The cold produced at the mixing chamber is what a quantum processor or detector is bolted to.
Why helium-3 is essential
No other substance offers a two-phase system with a finite solubility that survives to zero temperature. The residual solubility of helium-3 in superfluid helium-4, about 6.6 percent at absolute zero, is what makes continuous millikelvin operation possible. This is the physical reason a scarce isotope sits at the center of quantum-computing infrastructure, and why supply of helium-3 matters for the field.
- Base temperature: roughly 5-10 mK for standard commercial units
- Cooling power: tens to hundreds of microwatts at 100 mK
- Operation: continuous, closed-cycle recirculation