Cryostat Design
A cryostat is the nested-shield vacuum enclosure that stages temperature from room ambient down to millikelvin while blocking radiation and conduction.
The temperature staircase
A dilution-refrigerator cryostat is organized as a set of nested plates, each held at a progressively colder temperature: room temperature, a 50-kelvin stage, a 4-kelvin stage, a still stage near 0.7 kelvin, a cold plate near 0.1 kelvin, and the mixing-chamber plate at base temperature. Each stage intercepts heat before it can reach the colder stages below, so the cold end sees only a tiny residual load.
Fighting three heat paths
Heat reaches the cold stages by radiation, conduction, and convection. Convection is eliminated by evacuating the cryostat to high vacuum. Radiation is blocked by gold-plated or polished radiation shields anchored to the 50-kelvin and 4-kelvin stages, since radiative load scales as the fourth power of temperature and must be caught while it is warm. Conduction is managed by choosing support struts and wiring with low thermal conductivity and by heat-sinking everything at each stage.
Materials and mechanics
Plates are usually high-purity copper or gold-plated copper for good lateral thermal conductivity. Structural supports use low-conductivity, high-strength materials such as stainless steel or composites to hold the cold mass without shorting heat down to it. Everything must survive repeated thermal cycling between room temperature and millikelvin without loosening, since differential contraction is significant.
Magnetic and electromagnetic shielding
Superconducting qubits are sensitive to stray magnetic fields and radio-frequency interference. Cryostats add mu-metal or superconducting shields around the cold stages and carefully filtered, light-tight wiring so that only intended signals reach the device. The result is an enclosure that is simultaneously a thermos, a Faraday cage, and a magnetic shield.
- Nested plates at 50 K, 4 K, still, cold plate, and mixing chamber
- High vacuum kills convection
- Radiation shields anchored at warm stages block infrared load
- Magnetic and RF shielding protect qubits