Millikelvin Cooling for Qubits

Why qubits need temperatures a hundredth of a degree above absolute zero — and what that requires.

In brief

• Superconducting qubit energy gaps correspond to ~100–200 mK; operating well below that suppresses thermal excitation.
• Thermal photons and quasiparticles are dominant error sources; cold means quiet.
• Multi-stage cooling: pulse tube (~4 K) → still → mixing chamber (~10 mK).
• Every stage below ~1 K relies on the helium-3 dilution cycle.

The detail

A qubit is a two-level system with a tiny energy gap. If the surrounding temperature is comparable to that gap, random thermal energy flips the qubit and erases the computation. Keeping the environment at ~10–20 mK makes thermal excitation exponentially unlikely, protecting coherence. Reaching that regime and holding it under the heat load of control wiring is the job of the dilution refrigerator — and thus of helium-3.