“ It’s kind of hard to get rid of excess heat in a vacuum, Elon. But what do I know.”
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Elon says heat can be radiated away through a vacuum. The sun warms the Earth by radiating heat through almost 93 million miles of vacuum. Just sayin’.
That's true. But the sun's surface temperature is almost 6000 K. Radiative heat transfer is governed by some fairly demanding equations. At 6000 K, you can lose quite a lot of heat to the vacuum. At room temperature, the amount is a lot less (the amount radiated scales as the fourth power of absolute temperature).
Pulling one watt of thermal energy from quantum computing temperatures up to room temperature takes almost 3kW of power, with 100% mechanical efficiency. Actual practical cryocoolers are much less efficient than that, from 1 to 5%.
That means that it will take something on the order of 150kW of electrical power to remove one watt of thermal energy from a quantum computer.
Of course, that 150 kW has to be generated, stored, converted from one form to another, etc. That all generates more heat.
But, as I said, what do I know.
The cryocooler on the James Webb Space Telescope needs around 300 watts of power to lift 55 mW of heat from 6.2K to a hot side temperature of around 300 K, which is about 80° F.
Lifting the same amount of heat from 0.1 K would require much more power. And 0.1 K is the high limit for present day quantum computing circuitry.
But what do I know.