We are nowhere near "ubiquitous" quantum computing. We aren't near rare quantum computing.

Quantum computing as a practical platform has yet to be proven feasible. When you ask people who know what they're talking about and aren't pitching for grant money, quantum computing is somewhere between decades away [1] and never happening [2].

[1]: https://www.nature.com/articles/d41586-019-02936-3

[2]: https://spectrum.ieee.org/the-case-against-quantum-computing

"Never happening" is way too harsh imo, but it definitely is still decades away.

You need on the order of millions of qubits for quantum error correction algorithms to work.

We have, with superconducting circuits operating at 20 milli-kelvin, managed to corral 53 qubits into a circuit. In the error-correcting model, we must perform simultaneous gate operations on at least thousands of qubits. We have managed to perform simultaneous gate operations on two.

The levels of engineering effort required, and the orders of magnitude separating what has been realized by those efforts and what is required by theory, lends itself towards narratives of impossibility. Unlike the transistor revolution, there is no clear path forward upon which we might improve these initial results.

To quote my second source:

> I believe that, appearances to the contrary, the quantum-computing fervor is nearing its end. That’s because a few decades is the maximum lifetime of any big bubble in technology or science. After a certain period, too many unfulfilled promises have been made, and anyone who has been following the topic starts to get annoyed by further announcements of impending breakthroughs. What’s more, by that time all the tenured faculty positions in the field are already occupied. The proponents have grown older and less zealous, while the younger generation seeks something completely new and more likely to succeed.

> All these problems, as well as a few others I’ve not mentioned here, raise serious doubts about the future of quantum computing. There is a tremendous gap between the rudimentary but very hard experiments that have been carried out with a few qubits and the extremely developed quantum-computing theory, which relies on manipulating thousands to millions of qubits to calculate anything useful. That gap is not likely to be closed anytime soon.

> To my mind, quantum-computing researchers should still heed an admonition that IBM physicist Rolf Landauer made decades ago when the field heated up for the first time. He urged proponents of quantum computing to include in their publications a disclaimer along these lines: “This scheme, like all other schemes for quantum computation, relies on speculative technology, does not in its current form take into account all possible sources of noise, unreliability and manufacturing error, and probably will not work.”

Ok, as a scientist, when I say "never" is too harsh, what I mean by QCs becoming a thing probably isn't what most people on HN think of as QCs but rather being objects for simulating quantum systems. For that, they already have use (that is, those QC systems you keep hearing about on the news that already exist and are being used) and probably will get better to the point (god willing) we can simulate many electron systems. That is _my_ dream. I feel like QCs in HN minds is a lot more towards the "Computer" part of QC, like an actual Turing complete computer that will be able to do Shor's algorithm and break modern encryption, and on that I sort of agree with your assessment that it is between decades to never.

Sorry for that, I have to context switch when I talk to people outside physics. I always forget that. Also definitely the context of the convo was about QCs breaking encryption so my bad.

My favourite term for such disambiguation is Cryptographically Relevant Quantum Computers.

I am curious as to your perspective as a physicist, do you think it is feasible to have a QC computer from an energy perspective?

There is the cost to consider, yes, there is also an energy cost to a stable QC system. Asymmetric/symmetric are not unbeatable, they have an energy cost. Shors algorithm is theoretically great, but rarely if ever have I seen an associated energy cost...even outside of the answer "will we build one" the question is "can you efficiently build one" or not, i.e. what does a QC capable of executing shor's algorithm look like, a small planet or star perhaps?

So, as nickelpro states, I feel like a QC that is actually general purpose (which is probably a better way to state it) is so difficult at this point to even imagine, it's hard to say it will become a thing absent some breakthrough that is hereto unknown. You probably could state it as an energy cost thing by somehow deriving how much energy it would take to keep millions of quibits from decohering by extrapolating from how much energy it takes to keep a few from decohering, but I'm not even sure you can extrapolate that far out since as you increase the number of quibits the required energy probably isn't linearly related to the number of quibits but it is some power law or worse. Remember, the number of quibits we can run is in the dozens today, the numbers you need for Shor's algo or just general purpose computing is likely in the millions.

For quantum people, the QCs are already pretty cool because they can do simulations of quantum systems like molecules and atoms that are just infeasible on classical computing (high performance computing, ie. supercomputer) systems, things that would take probably years (yes years) of wall time on a HPC system. The thing is the number of required quibits for modeling these types of molecules is likely in the dozens to 100+ quibits, which looks possible now since there are systems out there that, while noisy, do have dozens of operating quibits.

If you're curious what these simulations are for, it's doing things like calculating energy levels for certain molecules, which materials science people care about and will help them make the next generation subtrate for a computer chips, etc etc. So it's not entirely esoteric stuff, it will be things which will eventually make it into actual products and technology people use, but it definitely is NOT general purpose computing, even less so Shor's algorithm or breaking encryption.