A dilution refrigerator uses six nested temperature stages — from 300 K at the top to 15 mK at the bottom — each progressively colder, each serving a distinct role in protecting the qubits.
When you route a cable from room temperature to the qubit, it crosses every stage. Each crossing is a chance for heat to leak in. Understanding the stage stack tells you where to place attenuators, where to anchor cables, and why certain components must go at specific temperatures.
You cannot jump from room temperature to 15 millikelvin in one step. Instead, a dilution refrigerator uses a series of nested plates, each colder than the one above. The 50 K stage intercepts the bulk of the heat from room-temperature cables. The 4 K stage is where helium liquefies and most active electronics (like amplifiers) operate. The Still, Cold Plate, and Mixing Chamber stages use the mixing of helium-3 and helium-4 isotopes to reach sub-kelvin temperatures. The qubits sit at the very bottom, shielded by every stage above them.
The cryostat is like a set of nested Russian dolls, each colder than the last. The outermost shell faces room temperature. Each inner shell is colder and has less cooling power. The innermost shell — the mixing chamber at 15 mK — can only remove microwatts of heat, so everything above it must intercept as much heat as possible before it reaches the bottom.
Think of the cryostat as a set of nested Russian dolls, each inside a colder shell. The outermost doll faces the full force of room-temperature heat. Each inner doll sees less and less thermal radiation. The innermost doll — the mixing chamber — experiences almost no heat at all, which is exactly what the qubits need.
The six stages in a typical dilution refrigerator are: 300 K (room temperature flange), 50 K (first radiation shield, pulse-tube cooler), 4 K (second radiation shield, pulse-tube second stage), Still (~800 mK, helium evaporation), Cold Plate (~100 mK, intermediate heat exchanger), and Mixing Chamber (~15 mK, helium-3/helium-4 phase boundary). Cooling power decreases at each stage: the 50 K stage can absorb tens of watts, while the mixing chamber provides only microwatts. This means every milliwatt of heat that reaches the coldest stages must be carefully intercepted or filtered out at warmer stages.
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