Real cryostats reach ultra-low temperatures by staging several unrelated cooling mechanisms end to end — each one taking over exactly where the previous method runs out of cooling power, spanning roughly ten orders of magnitude in temperature.
300 K → 4 K → 10 mK → 1 mK → 100 nK
mechanical → dilution → demagnetization → laser
- Mechanical pre-cooling (~300 K → 4 K) — pulse-tube / liquid-helium stages remove bulk heat by ordinary heat exchange, the same principle as a household fridge, down to the boiling point of liquid helium.
- Dilution refrigerator (~4 K → 10 mK) — mixing helium-3 into helium-4 below their phase-separation point costs entropy, exactly like evaporation costs latent heat; continuously forcing ³He across the phase boundary pulls heat out of the mixing chamber down into the millikelvin range. This is the workhorse of superconducting quantum computers.
- Adiabatic demagnetization (~10 mK → ~1 mK) — a paramagnetic salt is magnetized (aligning its spins, lowering spin entropy) while thermally linked to the cold bath, then thermally isolated and the field is switched off; the spins randomize again, and since total entropy can't drop, that entropy increase is paid for by absorbing heat from the lattice, cooling the sample further.
- Laser cooling (down to ~100 nK) — atoms are hit by laser light tuned slightly below an atomic transition; the Doppler effect means only atoms moving toward a beam absorb its photons efficiently, so every absorption is a tiny push against the atom's motion. Repeated millions of times per second, this optical molasses removes kinetic energy directly, reaching nanokelvin temperatures used in Bose–Einstein condensate experiments.
The temperature ladder on the right is drawn on a logarithmic scale — a straight visual distance near the top (K) compresses a much smaller physical temperature drop than the same distance near the bottom (nK), reflecting how much harder each successive decade is to reach.