https://scottaaronson.blog/?p=9425 Shtetl-Optimized The Blog of Scott Aaronson If you take nothing else from this blog: quantum computers won't solve hard problems instantly by just trying all solutions in parallel. Also, please read Zvi Mowshowitz's masterpiece on how to fix K-12 education! --------------------------------------------------------------------- << Happy Chanukah More on whether useful quantum computing is "imminent" These days, the most common question I get goes something like this: A decade ago, you told people that scalable quantum computing wasn't imminent. Now, though, you claim it plausibly is imminent. Why have you reversed yourself?? I appreciated the friend of mine who paraphrased this as follows: "A decade ago you said you were 35. Now you say you're 45. Explain yourself!" --------------------------------------------------------------------- A couple weeks ago, I was delighted to attend Q2B in Santa Clara, where I gave a keynote talk entitled "Why I Think Quantum Computing Works" (link goes to the PowerPoint slides). This is one of the most optimistic talks I've ever given. But mostly that's just because, uncharacteristically for me, here I gave short shrift to the challenge of broadening the class of problems that achieve huge quantum speedups, and just focused on the experimental milestones achieved over the past year. With every experimental milestone, the little voice in my head that asks "but what if Gil Kalai turned out to be right after all? what if scalable QC wasn't possible?" grows quieter, until now it can barely be heard. Going to Q2B was extremely helpful in giving me a sense of the current state of the field. Ryan Babbush gave a superb overview (I couldn't have improved a word) of the current status of quantum algorithms, while John Preskill's annual where-we-stand talk was "magisterial" as usual (that's the word I've long used for his talks), making mine look like just a warmup act for his. Meanwhile, Quantinuum took a victory lap, boasting of their recent successes in a way that I considered basically justified. --------------------------------------------------------------------- After returning from Q2B, I then did an hour-long podcast with "The Quantum Bull" on the topic "How Close Are We to Fault-Tolerant Quantum Computing?" You can watch it here: As far as I remember, this is the first YouTube interview I've ever done that concentrates entirely on the current state of the QC race, skipping any attempt to explain amplitudes, interference, and other basic concepts. Despite (or conceivably because?) of that, I'm happy with how this interview turned out. Watch if you want to know my detailed current views on hardware--as always, I recommend 2x speed. Or for those who don't have the half hour, a quick summary: * In quantum computing, there are the large companies and startups that might succeed or might fail, but are at least trying to solve the real technical problems, and some of them are making amazing progress. And then there are the companies that have optimized for doing IPOs, getting astronomical valuations, and selling a narrative to retail investors and governments about how quantum computing is poised to revolutionize optimization and machine learning and finance. Right now, I see these two sets of companies as almost entirely disjoint from each other. * The interview also contains my most direct condemnation yet of some of the wild misrepresentations that IonQ, in particular, has made to governments about what QC will be good for ("unlike AI, quantum computers won't hallucinate because they're deterministic!") * The two approaches that had the most impressive demonstrations in the past year are trapped ions (especially Quantinuum but also Oxford Ionics) and superconducting qubits (especially Google but also IBM), and perhaps also neutral atoms (especially QuEra but also Infleqtion and Atom Computing). * Contrary to a misconception that refuses to die, I haven't dramatically changed my views on any of these matters. As I have for a quarter century, I continue to profess a lot of confidence in the basic principles of quantum computing theory worked out in the mid-1990s, and I also continue to profess ignorance of exactly how many years it will take to realize those principles in the lab, and of which hardware approach will get there first. * But yeah, of course I update in response to developments on the ground, because it would be insane not to! And 2025 was clearly a year that met or exceeded my expectations on hardware, with multiple platforms now boasting >99.9% fidelity two-qubit gates, at or above the theoretical threshold for fault-tolerance. This year updated me in favor of taking more seriously the aggressive pronouncements--the "roadmaps"--of Google, Quantinuum, QuEra, PsiQuantum, and other companies about where they could be in 2028 or 2029. * One more time for those in the back: the main known applications of quantum computers remain (1) the simulation of quantum physics and chemistry themselves, (2) breaking a lot of currently deployed cryptography, and (3) eventually, achieving some modest benefits for optimization, machine learning, and other areas (but it will probably be a while before those modest benefits win out in practice). To be sure, the detailed list of quantum speedups expands over time (as new quantum algorithms get discovered) and also contracts over time (as some of the quantum algorithms get dequantized). But the list of known applications "from 30,000 feet" remains fairly close to what it was a quarter century ago, after you hack away the dense thickets of obfuscation and hype. --------------------------------------------------------------------- I'm going to close this post with a warning. When Frisch and Peierls wrote their now-famous memo in March 1940, estimating the mass of Uranium-235 that would be needed for a fission bomb, they didn't publish it in a journal, but communicated the result through military channels only. As recently as February 1939, Frisch and Meitner had published in Nature their theoretical explanation of recent experiments, showing that the uranium nucleus could fission when bombarded by neutrons. But by 1940, Frisch and Peierls realized that the time for open publication of these matters had passed. Similarly, at some point, the people doing detailed estimates of how many physical qubits and gates it'll take to break actually deployed cryptosystems using Shor's algorithm are going to stop publishing those estimates, if for no other reason than the risk of giving too much information to adversaries. Indeed, for all we know, that point may have been passed already. This is the clearest warning that I can offer in public right now about the urgency of migrating to post-quantum cryptosystems, a process that I'm grateful is already underway. --------------------------------------------------------------------- Update: Someone on Twitter who's "long $IONQ" says he'll be posting about and investigating me every day, never resting until UT Austin fires me, in order to punish me for slandering IonQ and other "pure play" SPAC IPO quantum companies. And also, because I've been anti-Trump and pro-Biden. He confabulates that I must be trying to profit from my stance (eg by shorting the companies I criticize), it being inconceivable to him that anyone would say anything purely because they care about what's true. Email, RSS Follow This entry was posted on Sunday, December 21st, 2025 at 11:34 am and is filed under Adventures in Meatspace, Quantum, Speaking Truth to Parallelism. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site. 13 Responses to "More on whether useful quantum computing is "imminent"" 1. Michael Marthaler Says: Comment #1 December 21st, 2025 at 1:32 pm Do you know if the Q2B Talks will also be online available? 2. Scott Says: Comment #2 December 21st, 2025 at 1:48 pm Michael #1: At least some of them, I think, but not sure when. 3. Soatok Says: Comment #3 December 21st, 2025 at 5:23 pm This is the clearest warning that I can offer in public right now about the urgency of migrating to post-quantum cryptosystems, a process that I'm grateful is already underway. I want to share something I read a while back: Quantum is unimportant to post-quantum. The author argues that adopting post-quantum cryptography is valuable even if it turns out that quantum computers are infeasible to build in our lifetimes. And, therefore, resting on one's laurels with the PQ rollout isn't a good idea even if you're highly skeptical. For my part, I've been advocating for X-Wing for ActivityPub E2EE efforts as the default KEM for encrypting private messages. 4. foo Says: Comment #4 December 21st, 2025 at 5:38 pm As a complete outsider, my takeaway is "I can ignore the hype about QC for a few more years", and I'll take that any day. Out of curiosity, do you have any opinion about French QC startups (Pasqal, Quandela)? They tend to have serious scientists as founders, but of course that's usually not enough. 5. Anon Says: Comment #5 December 22nd, 2025 at 9:17 am educating policy markers becomes more important as we get closer to useful quantum computers you may like the following talk. https://www.youtube.com/live/t5HBhE-Q5xA the speakers emphasized a few important aspects: 1. need more research into safety of the current post-quantum crypto mechanisms, we might not be on very solid grounds 2. need to make crypto based systems more modular so if we notice one is broken, we can quickly and easily replace it 3. need more funding for computer science research to find new quantum algorithms, to make quantum computers more useful (also in your talk, but I liked how they framed it not just as the statement of the current state but as an opportunity for an area that is not getting enough research funding compared to quantum hardware) the note that the number of operations that a quantum computer can perform will be much smaller than the current classical ones had a striking sharpness, I am going to use that when I try to explain that quantum computers are not really faster classical computers, but rather different beasts that are actually slower but can do new operations, and to make them actually useful we need new algorithms that exploit those new operations 6. Johnny D Says: Comment #6 December 22nd, 2025 at 11:27 am Scott, this is an argument for QC failure. This post seems like the place for it. Here is an argument that assumes quantum theory is correct and even fundamentally reality is a QC gauge theory. In quantum circuit gauge theory, the circuit has 2 types of size, number of logical qubits and amount of entanglement amongst the logical qubits. Fixing entanglement and increasing qubits can give EFT. Fixing qubits and increasing entanglement gives gravity with strength determined by the specifics of the theory. In the semantics of error correcting quantum computing, logical qubits exist in a background gauge theory. For universal QC, the logical qubits are defects in the gauge theory that are degenerate vacuum states. This means that in the semantic gauge theory, there is no cost to transport defects around each other to do computation. Thus the semantic gravitation emerges with zero strength and can be ignored even for highly entangled states. At the physical qubit level, states always have energy differentials. They are not degenerate states. That is what makes them stable enough to define. It takes energy to move entaninglement around. If reality is a circuit gauge theory then large entanglement amongst these physical qubit states do have gravitational effects with a nonzero strength. Why can this be ignored? Does it put an entanglement limit on QC? I think the effect would show up in the Born rule for the error correcting code as there would be spurious correlations not accounted for in the QECC. 7. Scott Says: Comment #7 December 22nd, 2025 at 11:41 am Johnny D #6: I was barely able to make sense of your argument--maybe someone else will have better luck than me. What I can tell you is that people have estimated gravitational sources of decoherence, and they are utterly negligible compared to more prosaic sources of decoherence. As in, absent some shocking new development in physics, your QC would need to literally be on astronomical scale before they were relevant--and even then, there's no reason why quantum error correction couldn't handle it the same as it handles any other small decoherence source. I don't see how you could get any other answer without a huge change to our understanding of QM. And certainly the experimental successes of the past couple years, which I covered in my talk and interview, have revealed no novel kinds of decoherence, only the prosaic kinds that we know how to handle, up to the scale of hundreds of qubits and thousands of operations. Do you have a prediction for the scale at which your conjectured new effect would become relevant? 8. Johnny D Says: Comment #8 December 22nd, 2025 at 12:20 pm Scott #7: This is not decoherence. I am assuming the QECC works perfectly to deal with decoherence. This is about how gravity emerges from quantum systems. This is about QECC semantics of vaccuum degeneracy not being a valid assumption at the physical qubits level where the entanglement syntactically (physically) is. I think the estimate would depend on energy differentials of the physical qubits. I will think about an estimate, but I wouldn't hold my breath. 9. Johnny D Says: Comment #9 December 22nd, 2025 at 12:33 pm Scott #7: Sorry, I didn't notice your comment on the understanding of quantum mechanics. This is not about quantum theory being wrong. This is about QECC ignoring this effect. 10. Scott Says: Comment #10 December 22nd, 2025 at 1:17 pm Johnny D #8: Alright then, if you ever come up with a more concrete prediction for what we'll see that differs from the standard predictions of QC theory (and when), feel free to share it here ... but I'll follow your instructions and not hold my breath! 11. Topics Everyone Is Talking About No337 - x321.org Says: Comment #11 December 22nd, 2025 at 1:40 pm [...] [?][?] Is Practical Quantum Computing Finally Near? A thoughtful and balanced reflection on the state of quantum computing--combining scientific rigor with a realistic view of its current limitations and potential. Scott Aaronson revisits the question of whether practical quantum computing is close at hand, following insights from the Q2B conference. He notes impressive progress from Google, Quantinuum, and QuEra, with qubit fidelity surpassing fault-tolerance thresholds. While confident in the robustness of quantum theory, he remains skeptical of overhyped claims and warns that analyses of cryptography-breaking potential may soon be restricted due to growing security risks. Read more [...] 12. Aha! Says: Comment #12 December 22nd, 2025 at 2:58 pm "unlike AI, quantum computers won't hallucinate because they're deterministic" A complexity theorist saying QC are deterministic is a blasphemy of highest order. 13. Scott Says: Comment #13 December 22nd, 2025 at 3:45 pm Aha! #12: Can you read? Di Masi, who I was quoting, is the CEO of IonQ, not a complexity theorist. Leave a Reply You can use rich HTML in comments! You can also use basic TeX, by enclosing it within $$ $$ for displayed equations or \( \) for inline equations. Comment Policies: After two decades of mostly-open comments, in July 2024 Shtetl-Optimized transitioned to the following policy: All comments are treated, by default, as personal missives to me, Scott Aaronson---with no expectation either that they'll appear on the blog or that I'll reply to them. At my leisure and discretion, and in consultation with the Shtetl-Optimized Committee of Guardians, I'll put on the blog a curated selection of comments that I judge to be particularly interesting or to move the topic forward, and I'll do my best to answer those. But it will be more like Letters to the Editor. Anyone who feels unjustly censored is welcome to the rest of the Internet. To the many who've asked me for this over the years, you're welcome! 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