Regarding the coaxial cables, you seem to be an expert, so tell me if I'm wrong, but it seems to me a limitation of current designs (and in particular of superconducting qubits), I don't think there is any fundamental reason why this could not be replaced by a different tech in the future. Plus, the scaling must not need to be infinite, right? Even with current "coaxial cable tech", it "only" needs to scale up to the point of reaching one logical qubit.
The QC is designed with coaxial cables running from the physical qubits outside the cryostat because the pulse measurement apparatus is most precise in large, bulky boxes. When you miniaturise it for placement next to qubits, you lose precision, which increases the error rate.
I am not even sure whether logical components work at such low temperatures, since everything becomes superconducting.
> Even with current "coaxial cable tech", it "only" needs to scale up to the point of reaching one logical qubit.
Having a logical qubit sitting in a big box is insufficient. One needs multiple logical qubits that can be interacted with and put in a superposition, for example. A chain of gates represents each logical qubit gate between each pair of physical qubits, but that's not possible to do directly at once; hence, one needs to effectively solve the 15th puzzle with the fewest steps so that the qubits don't decohere in the meantime.
Currently finishing a course where the final project is designing a semiconductor (quantum dot) based quantum computer. Obviously not mature tech yet, but we've been stressed during the course that you can build most of the control and readout circuits to work at cryogenic temps (2-4K) using slvtfets. The theoretical limit for this quantum computing platform is, I believe, on the order of a million qubits in a single cryostat.
Given the magic that happens inside high-precision control and readout boxes connected to qubits with coaxial cables, I would not equate the possibility of building one with such a control circuit ever reaching the same level of precision. I find it strange that I haven’t seen that on the agenda for QC, where instead I see that multiplexing is being used.
> The theoretical limit for this quantum computing platform is, I believe, on the order of a million qubits in a single cryostat.
What are the constraints here?
Have you ever wondered what will happen to those coaxial cables seen in every quantum computer setup, which scale approximately linearly with the number of physical qubits? Multiplexing is not really an option when the qubit waiting for its control signal decoheres in the meantime.