So how many gates are we talking to factor some "cryptographically useful" number? Is there some pathway that makes quantum computers useful this century?
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As a layman the pathway seems to exist behind multiple massive materials science breakthroughs
That is a hard question to answer for two reasons. First, there is no bright line that delineates "cryptographically useful". And second, the exact design of a QC that could do such a calculation is not yet known. It's kind of like trying to estimate how many traditional gates would be needed to build a "semantically useful" neural network back in 1985.
But the answer is almost certainly in the millions.
[UPDATE] There is a third reason this is hard to predict: for quantum error correction, there is a tradeoff between the error rate in the raw qbit and the number of gates needed to build a reliable error-corrected virtual qbit. The lower the error rate in the raw qbit, the fewer gates are needed. And there is no way to know at this point what kind of raw error rates can be achieved.
> Is there some pathway that makes quantum computers useful this century?
This century has 75 years left in it, and that is an eternity in tech-time. 75 years ago the state of the art in classical computers was (I'll be generous here) the Univac [1]. Figuring out how much less powerful it was than a modern computer makes an interesting exercise, especially if you do it in terms of ops/watt. I haven't done the math, but it's many, many, many orders of magnitude. If the same progress can be achieved in quantum computing, then pre-quantum encryption is definitely toast by 2100. And it pretty much took only one breakthrough, the transistor, to achieve the improvement in classical computing that we enjoy today. We still don't have the equivalent of that for QC, but who knows when or if it will happen. Everything seems impossible until someone figures it out for the first time.
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[1] https://en.wikipedia.org/wiki/UNIVAC_I#Technical_description
You can do useful and valuable quantum chemistry calculations already with few 100s of qubits with that low error rates, while post-quantum algorithms are becoming more common everyday removing incentives to build crypto cracking quantum computers.
I think the quantum computing will advance fastest in directions that are not easy to use in cryptography.
> This century has 75 years left in it, and that is an eternity in tech-time.
As a comparison, we went from first heavier than air flight to man walking on the moon in only 66 years.
Note that the magic of quantum error correction (exponential improvement in the error rate goes both ways): if you could get another 9 in qubit fidelity, you get a much larger improvement in qubit numbers. On the other hand, if you need to split your computation over several systems, things get much worse.
Table 5 of [1] estimates 7 billion Toffoli gates to factor 2048 bit RSA integers.
> Is there some pathway that makes quantum computers useful this century?
The pathway to doing billions of gates is quantum error correction. [1] estimates distance 25 surface codes would be sufficient for those 7 billion gates (given the physical assumptions it lists). This amplifies the qubit count from 1400 logical qubits to a million physical noisy qubits.
Samuel Jacques had a pretty good talk at PQCrypto this year, and he speculates about timelines in it [2].
(I'm the author of this blog post and of [1].)
And that was before Epoch (1969, unix time started in 1970). We went from calculator to AI in 55 years, which is, actually, extremely long. It took exactly the time to miniaturize CPUs enough that you would hold as many gates in a GPU as neurones in a human’s brain. The moment we could give enough transistors to a single program, AI appeared. It’s like it’s just an emergent behavior.
On the other hand, the Univac could do more useful work than current quantum computers.
Related to your observation: A piece of the original Wright Flyer was landed on Mars just a bit over 117 years after the first flight.
What made computing-at-scale possible wasn't the transistor, it was the precursor technologies that made transistor manufacturing possible - precise control of semiconductor doping, and precision optical lithography.
Without those the transistor would have remained a lab curiosity.
QC has no hint of any equivalent breakthrough tech waiting to kick start a revolution. There are plenty of maybe-perhaps technologies like Diamond Defects and Photonics, but packing density and connectivity are always going to be huge problems, in addition to noise and error rate issues.
Basically you need high densities to do anything truly useful, but error rates have to go down as packing densities go up - which is stretching optimism a little.
Silicon is a very forgiving technology in comparison. As long as your logic levels have a decent headroom over the noise floor, and you allow for switching transients (...the hard part) your circuit will be deterministic and you can keep packing more and more circuitry into smaller and smaller spaces. (Subject to lithography precision.)
Of course it's not that simple, but it is basically just extremely complex and sophisticated plumbing of electron flows.
Current takes on QC are the opposite. There's a lot more noise than signal, and adding more complexity makes the problem worse in non-linear ways.
I think it is insanely fast.
Think about it: that planet has been here for billions of years. Modern humanity has been here for 200,000 years, give or take. It took 199700 years and change to get to a working steam engine. 266 years later men were walking on the moon and another 55 years and we had a good facsimile of what an AI looks like in practice. That's insane progress. The next 75 years are going to be very interesting, assuming we don't fuck it all up, the chances of which are right now probably 50/50 or so.
Yet it has been 53 years since we have been able to send a manned mission to the moon . No other program has or likely to come close in the next 13 years including the current US one. By 2038 the moon landings would be closer to Wright brothers than future us.
The curve of progress is only smooth and exponential when you squint hard .
It is a narrow few decades of exponential growth hardly can reasonably be expected to last for 100+ years .
It is for the same reason you cannot keep doubling grains on a chess board just because you did it 10-20 steps quickly.
Fusion power, quantum computing are all always two decades away for a reason despite the money being spent . AI has gone through 3-4 golden ages in living memory and yet too many keep believing this one would last.
Reality is when the conditions are right, I.e. all the ground work has been done for decades or centuries before there can be rapid innovation for a short(few decades at best) time
In some special problems hybrid methods start giving gains in 100 qubits or below.
Gate count estimates for performing quantum chemistry on small quantum computers https://arxiv.org/pdf/1312.1695
A Perspective on Quantum Computing Applications in Quantum Chemistry using 25--100 Logical Qubits https://arxiv.org/pdf/2506.19337
Science fiction has been predicting what an AI would be like for over a hundred years, there was even one in a movie in 1927. We're so far from what we dream that, to me, it feels like a mere leaf blowing in the wind compared to the Wright Flyer.
The Wright Flyer was a complete aircraft but small, awkward and not very practical. But it had all of the parts and that was the bit that mattered.
LLMs are not a 'complete AI' at all, they are just a very slick imitation of one through a completely different pathway. Useful, but not AI (at least, not to me). Meanwhile, a very large fraction of the users of OpenAI, Claude etc all think that AI has arrived and from that perspective it is mostly the tech crowd that is disappointed. For the rest of the people the thing is nothing short of magic compared to what they were able to do with a computer not so long ago. And for people like translators it is a massive threat to their jobs, assuming they still have one.
It is both revolutionary and a letdown, depending on your viewpoint and expectations.
The Chinese are planning manned lunar landings in 2029-2030, and this is not a pipe dream, they've been systematically working at this for several decades now. They have already completed 6 out of 8 preparatory missions plus placed comms satellites in lunar orbit, and the final two are scheduled for 2026 and 2028.
https://en.wikipedia.org/wiki/Chinese_Lunar_Exploration_Prog...
Sci-fi is fanciful and doesn’t take into account psychology. What we got is the local maxima of what entrepreneurs think they can build and what people are willing to pay for.
Sci-fi is not a prediction. It is a hypothetical vision for what humanity could be in a distant future. The writer doesn’t have to grapple with limitations of physics (note FTL travel is frequently a plot device, not a plausible technology) or limitations about what product-market-fit the market will adopt.
And, of course, sci-fi dates are rarely close or accurate. That’s probably by design (most Star Trek space technologies would be unbelievable if the timeline was 2030, but more easily believable if you add a few thousand years for innovation).
Indeed, and at the same the breakthroughs are vastly outnumbered by ideas which had plausible sounding counterarguments which turned out to be correct. Which is to say, the burden of proof is on the people making claims that something implausible-sounding is plausible.
Meanwhile, even after the infamous LK-99 fiasco (which gripped this forum almost more than anywhere else) was exposed as an overblown nothingburger, I still had seemingly-intelligent people telling me with all seriousness that the superconductor breakthrough had a 50% chance of happening within the next year. People are absolutely, terminally terrible at estimating the odds of future events that are surrounded by hype.
There was some decent attempts at the turing test given limited subject matter long before LLM’s. As in people looking at the conversation where unsure if one of the parties was a computer. It’s really interesting to read some of those transcripts.
LLM’s actually score worse one some of those tests. Of course they do a huge range of other things, but it’s worth understanding both their strengths and many weaknesses.
A near total lack of demand explains that impressive stall.
Even if the shuttle had worked out as well as its designers hoped, was envisioned as a major retreat, while sucking all the dollars out of the room.
And today, the market for lunar landings is still very small.
I think what it shows is that many technologies might have come earlier from a research and development standpoint, if we had enough money to burn. But that was an unusual situation.
The operations all consist of saying, connect these 3 bits and do a reversible operation on them all together. Same as assembly, "add these two registers and store the sum in the first one..." You didn't need to introduce any new bits.
You only need to introduce new bits for steps that cannot be reversibly done, in assembly you get around this by being able to overwrite a register: in quantum, that requires an explicit measurement in the computational basis to figure out how you want to do stuff to zero it; zeroing a bit is not a unitary operation. But if you can encode the circuit in Toffoli gates which are perfectly reversible, you don't have to delete any bits after that encoding (but you may have to introduce extra bits to get to that encoding, like using Toffoli to build “x AND y” requires an extra z bit that effectively gets discarded at the end of the computation when everything is done and nobody cares what that bit holds, but it holds the information you would need to reverse that logical AND).
But yeah it's just number of operations that you need to run the algorithm, versus the number of registers that you need to run the algorithm, they're just two different numbers.
It is not like Fusion or Quantum Computing has lacked serious or continuous funding over the last 20-30 years.
Foundational model development is a classic current example. The returns are diminishing significantly, despite the tens of billions each quarter being thrown at the problem.
No other R&D effort in our history has this much resources being allocated to it, perhaps including even the Moon landings.
However the ability to allocate resources has limits. Big tech can spend few hundred billion a year a number that would have been unimaginable even a decade ago, but even they cannot spend few trillion dollars a year.
Perhaps milestones are being set to be competing with Artemis. When NASA gets delayed or reduced in scope, CNSA might reset to more achievable date.
That is just engineering risk on dates, there are other class of risks in geopolitics or economics etc.
Bottom line I am skeptical that a successful landing and return can be attempted in 2030. 2035 is a more realistic target I think.
As a trekkie this was a dream come true.
Unfortunately we still don't have a tricorder yet (despite Elisabeth Holmes' promise).
But we do have the apps and the games, they didn't have these in star trek. My phone is loaded with these (apps, not games)
If I were doing this work, I'd look at a rich virtual environment like Minecraft or simcity or something like that. But it could also be coq or a code development environment.
The big thing that could change the numbers is more reliable qbits. Most of the calculations so far are done with qbits right at the edge of where error correction works (about 5x better than current qbits). if you get another 10x in qbit quality you probably drop the required qbits by ~100-1000x.
I'm not trying to say anything about whether or not a CRQC will ever be built. I'm also not trying to say that pursuing PQC in the short term is a bad idea. But what I am saying is that the burden of proof remains on the believers to show that the engineering challenges are more than theoretically surmountable.
The result is that, if you keep adding qubits that can be operated on in parallel, Shor's algorithm basically just keeps getting faster and faster and faster. The energy cost doesn't go down, and the number of qubits required becomes frankly absurd, but the time can go really really low.