Cryogenics and Dilution Refrigeration
Superconducting and many spin qubits operate near 10 millikelvin, reached by a dilution refrigerator that exploits helium isotope mixing.
Why so cold
A superconducting qubit at 5 GHz has an energy quantum corresponding to about 0.24 kelvin. To keep the qubit in its ground state and free of thermal excitations, the environment must be far colder than this, typically around 10 to 20 millikelvin. Only a dilution refrigerator reaches such temperatures continuously.
How dilution refrigeration works
Below about 0.87 kelvin, a mixture of helium-3 and helium-4 separates into two phases: a helium-3-rich phase floating on a helium-3-dilute phase. Forcing helium-3 atoms across the phase boundary into the dilute phase absorbs heat, much as evaporation cools a liquid, but it continues down to a few millikelvin. Circulating helium-3 through a still and heat exchangers sustains this cooling indefinitely.
The stages
- A pulse-tube cooler reaching about 4 kelvin and 50 kelvin plates
- A still around 0.7 kelvin
- A cold plate near 0.1 kelvin
- The mixing chamber at roughly 10 millikelvin, where the qubits sit
Constraints on scaling
Each colder stage removes far less heat than the one above it; the mixing chamber may handle only hundreds of microwatts. Every control and readout line carries heat down, so the number of wires that can enter is limited by the fridge's cooling power. This thermal budget, not just chip fabrication, is a genuine ceiling on how many qubits one refrigerator can support, driving interest in cryogenic control electronics and multiplexing.
Trapped-ion and photonic machines avoid the coldest stages for the qubits themselves, though single-photon detectors and some optics still need cryogenics.