Go further · Chapter 9 · 9a · Real-world case
Why are quantum computers so cold, and how do they get there?
The short answer (tap to open)
Superconducting qubits sit inside a dilution refrigerator, the golden "chandelier". It cools in stages, from room temperature down to about a hundredth of a degree above absolute zero, using a mixture of two kinds of helium. It must be that cold because any heat would knock the qubits out of their states. The chip sits at the bottom.
Why so cold?
A superconducting qubit's two levels sit a tiny energy apart, matching microwaves of a few billion cycles a second (chapter 1's ladder). Heat is a constant jiggling of energy. At room temperature that jiggling is thousands of times bigger than the gap, and the qubit would flip at random constantly. At a hundredth of a degree above absolute zero, it's far smaller than the gap, and the qubit stays put. The metal also needs the cold to superconduct.
How does the fridge get there?
It cools in stages, each plate colder than the one above, hanging like a chandelier:
- about 50 degrees above absolute zero, then about 4, using pumped helium gas;
- about 1 degree;
- about a tenth of a degree;
- at the bottom, the mixing chamber, about a hundredth of a degree.
The last step uses a mixture of helium-3 (a rare form of helium) and ordinary helium-4. When helium-3 atoms cross from one layer of the mixture into the other, they soak up heat, a bit like sweat evaporating cools your skin. It runs continuously for months.
The top
Room temperature
Cables enter here and run all the way down. Every one carries heat with it.
The stops, in a list
- The top (Room temperature). Cables enter here and run all the way down. Every one carries heat with it.
- The first plate (About 50 degrees above absolute zero). Pumped helium gas does the first cooling.
- The second plate (About 4 degrees above absolute zero). Still pumped helium gas. The cables pass through filters here to strip out heat and noise. Colder than the surface of Pluto.
- The third plate (About 1 degree above absolute zero). Another stage soaking up heat before it reaches the plates below. Colder than outer space.
- The fourth plate (About a tenth of a degree above absolute zero). One more barrier between the warm world and the chip.
- The mixing chamber (About a hundredth of a degree above absolute zero). Helium-3 atoms cross from one layer of the mixture into the other and soak up heat, a bit like sweat evaporating cools your skin.
- The chip (About a hundredth of a degree above absolute zero). The size of a fingernail, finally getting the quiet it needs.
What are all those cables?
Each qubit needs control and readout lines running from room temperature to the bottom. Every cable carries heat and noise down with it, so each passes through filters and attenuators at each stage. Readout signals are so faint they need special quantum-limited amplifiers near the chip. Wiring is one of the main obstacles to scaling up to many thousands of qubits, and why some companies are building much larger fridges.
Any bottlenecks?
Helium-3 is scarce, produced mostly as a by-product of nuclear programmes, so its supply limits how many fridges the world can build. The fridges also take days to cool down and warm up.
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The chandelier is a staircase down to near absolute zero.
At the bottom, a fingernail-sized chip finally gets the quiet it needs.