Silicon qubits could use either electron spin or charge degree of freedom. I guess you assumed both are Ok.

Where is the "none of those" option? :-D Or will this resolve NA at some point if no succeed?

@MartinModrak There are a some others architectures like Majorana qubits (which at this point haven't been demonstrated) or the dark horse candidate electrons floating on liquid Helium, but I've focused on the biggest contenders here. If another architecture achieves the success or no architectures does then it will be a N/A.

@GavinBrennen Then you might want to specify a (possibly distant) cutoff date or another criteria when this will resolve NA (e.g. "when the scientific consensus is that breaking [your def] is not feasible with quantum comp")

@GavinBrennen I think this is 99%+ likely to be an N/A, but if you make a market asking whether any quantum computing technology will succeed at that by 2033, I might vote in it.