The cosmic microwave background makes even deep intergalactic space warmer (3 or 4 K) than the inside of Earth’s best fridges (the best as low as nanoK). 3 or 4 K is easy to achieve using liquid helium cooling, which is presumably why eg. Google’s Sycamore chip operators in that range. But achieving those low temperatures is a regular occurrence in labs all over the world—-you call AirGas or somebody and they deliver a dewar of liquid He.

And, close to a star, you have several neutrinos flying through your experiment. You have vibrations from earthquakes. You have electromagnetic coupling. etc, etc.

The temperatures are easy enough; you just have to compensate for thermal noise. The isolation isn't; without isolation, your signal is wrong.

But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation. So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.

> But unless you're doing a computation without I/O (i.e. no reading back the results, no providing inputs), you need coupling into the rest of the universe, and typically into a low-entropy part of the universe like the Earth, where you have entities that care about the computation.

With a quantum computer, you can only do that at the end of the computation. Not half-way through; that'll cause the computer to start doing a different (unwanted) computation instead. While the calculation is happening, you need (a high probability of) total isolation from the rest of the universe, so that the intermediate state of the computer only interferes with itself.

> So, I am not convinced having a deeply isolated part of the universe is the answer; in fact, the isolation vs. signal quality tradeoff makes it sound more and more like a fundamental limitation of practical concern.

It is a fundamental limitation of practical concern! Just like the need to keep conventional computer processors cool or they melt, or the fundamental limitations on the bandwidth that a radio frequency can give you. The people who deal with these limitations are called engineers.