Shtetl-Optimized

Part of me feels bad not to have written for weeks about quantum error-correction or BQP or QMA or even the new Austin-based startup that launched a “quantum computing dating app” (which, before anyone asks, is 100% as gimmicky and pointless as it sounds).

But the truth is that, even if you cared narrowly about quantum computing, there would be no bigger story right now than the fate of American science as a whole, which for the past couple weeks has had a knife to its throat.

Last week, after I blogged about the freeze in all American federal science funding (which has since been lifted by a judge’s order), a Trump-supporting commenter named Kyle had this to say:

No, these funding cuts are not permanent. He is only cutting funds until his staff can identify which money is going to the communists and the wokes. If you aren’t a woke or a communist, you have nothing to fear.

Read that one more time: “If you aren’t woke or a communist, you have nothing to fear.”

Can you predict what happened barely a week later? Science magazine now reports that the Trump/Musk/DOGE administration is planning to cut the National Science Foundation’s annual budget from $9 billion to only $3 billion (Biden, by contrast, had proposed an increase to $10 billion). Other brilliant ideas under discussion, according to the article, are to use AI to evaluate the grant proposals (!), and to shift the little NSF funding that remains from universities to private companies.

To be clear: in the United States, NSF is the only government agency whose central mission is curiosity-driven basic research—not that other agencies like DOE or NIH or NOAA, which also fund basic research, are safe from the chopping block either.

Maybe Congress, where support for basic science has long been bipartisan, will at some point grow some balls and push back on this. If not, though: does anyone seriously believe that you can cut the NSF’s budget by two-thirds while targeting only “woke communism”? That this won’t decimate the global preeminence of American universities in math, physics, computer science, astronomy, genetics, neuroscience, and more—preeminence that took a century to build?

Or does anyone think that I, for example, am a “woke communist”? I, the old-fashioned Enlightenment liberal who repeatedly risked his reputation to criticize “woke communism,” who the “woke communists” denounced when they noticed him at all, and who narrowly survived a major woke cancellation attempt a decade ago? Alas, I doubt any of that will save me: I presumably won’t be able to get NSF grants either under this new regime. Nor will my hundreds of brilliant academic colleagues, who’ve done what they can to make sure the center of quantum computing research remains in America rather than China or anywhere else.

I of course have no hope that the “Kyles” of the world will ever apologize to me for their prediction, their promise, being so dramatically wrong. But here’s my plea to Elon Musk, J. D. Vance, Joe Lonsdale, Curtis Yarvin, the DOGE boys, and all the readers of this blog who are connected to their circle: please prove me wrong, and prove Kyle right.

Please preserve and increase the NSF’s budget, after you’ve cleansed it of “woke communism” as you see fit. For all I care, add a line item to the budget for studying how to build rockets that are even bigger, louder, and more phallic.

But if you won’t save the NSF and the other basic research agencies—well hey, you’re the ones who now control the world’s nuclear-armed superpower, not me. But don’t you dare bullshit me about how you did all this so that merit-based science could once again flourish, like in the days of Newton and Gauss, finally free from meddling bureaucrats and woke diversity hires. You’d then just be another in history’s endless litany of conquering bullies, destroying what they can’t understand, no more interesting than all the previous bullies.

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1. Trump’s proposal for the US to “take over” Gaza and expel its inhabitants is, like nearly everything else Trump has said and done over the past two weeks and indeed the past decade, completely batshit insane.

2. As with countless other Trump proposals, I don’t see that it will actually happen — both because most Gazans will refuse to leave, and because Arab countries will refuse to take them.

3. I wonder whether all the anti-Israel activists in the US who withheld their vote (or even switched to Trump) to punish Biden and Harris for their support of Israel, are now happy with what they’ve gotten.

4. The solution has always been for some government to develop Gaza for the benefit of its inhabitants, rather than as a terror-base for attacking Israel. Hamas and UNRWA have shown that they’ll never do that. But the postwar administration of Germany and Japan demonstrates what’s possible in one generation if the will exists.

5. I wish the anti-Israel people would join me in demanding that. They ought to reflect that, if their only counteroffer is “Israel gets eradicated and its Jews return to the countries that murdered or expelled their families,” then they’re demanding something even more fantastical than Trump’s proposal.

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A few years ago, scientists feared they’d lose their jobs if they said anything against diversity programs.

I was against that.

Now scientists fear they’ll lose their jobs if they say anything for diversity programs.

I’m against that too.

A few years ago, if you didn’t list your pronouns, you were on the wrong side of history.

I was on the wrong side of history.

Now, if you want equal rights for your trans friends, you’re an enemy of the people.

I’m an enemy of the people.

Then, they said the woke triumph over universities, the media, and Silicon Valley had bent the moral arc of the universe and overrode individual conscience.

I chose conscience anyway.

Now they say the MAGA triumph over the White House, Congress, the Supreme Court, and (again) Silicon Valley has bent the moral arc back.

I choose conscience again.

Then and now the ideologues say: don’t you realize you need to pick a side?

What they don’t understand is that I have picked a side.

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It’s been almost impossible to get reliable information this week, but here’s what my sources are telling me:

There is still a complete freeze on money being disbursed from the US National Science Foundation. Well, there’s total chaos in the federal government much more broadly, a lot of it more immediately consequential than the science freeze, but I’ll stick for now to my little corner of the universe.

The funding freeze has continued today, despite the fact that Trump supposedly rescinded it yesterday after a mass backlash. Basically, program directors remain in a state of confusion, paralysis, and fear. Where laws passed by Congress order them to do one thing, but the new Executive Orders seem to order the opposite, they’re simply doing nothing, waiting for clarification, and hoping to preserve their jobs.

Hopefully the funding will restart in a matter of days, after NSF and other agencies go through and cancel any expense that can be construed as DEI-related. Hopefully this will be like the short-lived Muslim travel ban of 2017: a “shock-and-awe” authoritarian diktat that thrills the base but quickly melts on contact with the reality of how our civilization works.

The alternative is painful to contemplate. If the current freeze drags on for months, tens of thousands of grad students and postdocs will no longer get stipends, and will be forced to quit. Basic science in the US will essentially grind to a halt—and even if it eventually restarts, an entire cohort of young physicists, mathematicians, and biologists will have been lost, while China and other countries race ahead in those fields.

Also, even if the funding does restart, the NSF and other federal agencies are now under an indefinite hiring freeze. If not quickly lifted, this will shrink these agencies and cripple their ability to carry out their missions.

If you voted for Trump, because you wanted to take a hammer to the woke deep state or whatever, then please understand: you may or may not have realized you were voting for this, exactly, but this is what you’ve gotten. In place of professionals who you dislike and who are sometimes systematically wrong, the American spaceship is now being piloted by drunken baboons, mashing the controls to see what happens. I hope you like the result.

Meanwhile, to anyone inside or outside the NSF who has more information about this rapidly-evolving crisis: I strongly encourage you to share whatever you know in the comments section. Or get in touch with me by email. I’ll of course respect all wishes for anonymity, and I won’t share anything without permission. But you now have a chance—some might even say an enviable chance—to put your loyalty to science and your country above your fear of a bully.

Update: By request, you can also contact me at ScottAaronson.49 on the encrypted messaging app Signal.

Another update: Maybe I should’ve expected this, but people are now sending me Signal messages to ask quantum mechanics questions or share their views on random topics! Should’ve added: I’m specifically interested in on-the-ground intel, from anyone who has it, about the current freeze in American science funding.

Yet another update: Terry Tao discusses the NSF funding crisis in terms of mean field theory.

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Update: In the comments, Craig Gidney points out that Ronit’s O(n log² n) quantum circuits for the exact QFT were already published by Cleve and Watrous in 2000 (in a paper whose main point was something else, parallelization). Ronit’s O(n (log log n)²) circuits for the approximate QFT still appear to be new (Gidney says he and others knew related techniques but had never explicitly combined them). Of course, while the exact result was Platonically “known,” it wasn’t sufficiently well known that any of the four quantum algorithms experts I’d consulted had heard of it! Hopefully this very post will go some way toward fixing the situation.

Another Update: Richard Cleve writes in to say that the approximate QFT circuits were known also—albeit, in an unpublished 2-page abstract by Ahokas, Hales, and himself from the 2003 ERATO conference, as well as a followup Master’s thesis by Ahokas. Unlike with the exact case, I’m not kicking myself trying to understand how I missed these.

Ironically, I hope this post helps get this prior work a well-deserved mention when the QFT is covered in introductory quantum information classes.

Meanwhile, my hope that Ronit returns to do more theory is undiminished! When I was a kid, I too started by rediscovering things (like the integral for the length of a curve) that were centuries old, then rediscovering things (like an efficient algorithm for isotonic regression) that were decades old, then rediscovering things (like BQP⊆PP) that were about a year old … until I finally started discovering things (like the collision lower bound) that were zero years old. This is the way.

In my last post, I tried to nudge the arc of history back onto the narrow path of reasoned dialogue, walking the mile-high tightrope between shrill, unsupported accusation and naïve moral blindness. For my trouble, I was condemned about equally by leftists for my right-wing sympathies and by rightists for my left-wing ones. So today, I’ll ignore the fate of civilization and return to quantum computing theory: a subject that’s reliably brought joy to my life for a quarter-century, and still does, even as my abilities fade. It turns out there is a consolation for advancing age and senility, and it’s called “students.”

This fall, I returned from my two-year leave at OpenAI to teach my undergrad Introduction to Quantum Information Science course at UT Austin. This course doesn’t pretend to bring students all the way to the research frontier, and yet sometimes it’s done so anyway. It was in my first offering of Intro to QIS, eight years ago, that I encountered the then 17-year-old Ewin Tang, who broke the curve and then wanted an independent study project. So I gave her the problem of proving that the Kerenidis-Prakash quantum algorithm achieves an exponential speedup over any classical algorithm for the same task, not expecting anything to come of it. But after a year of work, Ewin refuted my conjecture by dequantizing the K-P algorithm—a breakthrough that led to the demolition of many other hopes for quantum machine learning. (Demolishing people’s hopes? In complexity theory, we call that a proud day’s work.)

Today I’m delighted to announce that my undergrad quantum course has led to another quantum advance. One day, after my lecture, a junior named Ronit Shah came to me with an idea for how best to distinguish three possible states of a qubit, rather than only two. For some reason I didn’t think much of it at the time, even though it would later turn out that Ronit had essentially rediscovered the concept of POVMs, the Pretty Good Measurement (PGM), and the 2002 theorem that the PGM is optimal for distinguishing sets of states subject to a transitive group action.

Later, after I’d lectured about Shor’s algorithm, and one of its centerpieces, the O(n²)-gate recursive circuit for the Quantum Fourier Transform, Ronit struck a second time. He told me it should be possible to give a smaller circuit by recursively reducing the n-qubit QFT to two (n/2)-qubit QFTs, rather than to a single (n-1)-qubit QFT.

This was surely just a trivial confusion, perfectly excusable in an undergrad. Did Ronit perhaps not realize that an n-qubit unitary is actually a 2ⁿ×2ⁿ matrix, so he was proposing to pass directly from 2ⁿ×2ⁿ to 2^n/2×2^n/2, rather than to 2^n-1×2^n-1?

No, he said, he understood that perfectly well. He still thought the plan would work. Then he emailed me a writeup—claiming to implement the exact n-qubit QFT in O(n log²n) gates, the first-ever improvement over O(n²), and also the approximate n-qubit QFT in O(n (log log n)²) gates, the first-ever improvement over O(n log n). He used fast integer multiplication algorithms to make the new recursions work.

At that point, I did something I’m still ashamed of: I sat on Ronit’s writeup for three weeks. When I at last dug it out of my inbox and read it, I could discover no reason why it was wrong, or unoriginal, or unimportant. But I didn’t trust myself, so with Ronit’s permission I sent the work to some of my oldest quantum friends: Ronald de Wolf, Cris Moore, Andrew Childs, and Wim van Dam. They agreed, after some back-and-forth, that the new circuits looked legit. A keystone of Shor’s algorithm, of quantum computing itself, and of my undergrad class had seen its first real improvement since 1994.

Last night Ronit’s paper appeared on the arXiv where you can read it.

In case anyone asks: no, this probably has no practical implication for speeding up factoring on a quantum computer, since the QFT wasn’t the expensive part of Shor’s algorithm anyway—that’s the modular exponentiation—and also, the O(n log n) approximate QFT would already have been used in practice. But it’s conceivable that Ronit’s circuits could speed up other practical quantum computing tasks! And no, we have no idea what’s the ultimate limit here, as usual in circuit complexity. Could the exact n-qubit QFT even be doable in O(n) gates?

I’d love for Ronit to continue in quantum computing theory. But in what’s surely a sign of the times, he’s just gone on leave from UT to intern at an AI hardware startup. I hope he returns and does some more theory, but if he doesn’t, I’m grateful that he shared this little gem with us on his way to more world-changing endeavors.

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Did Elon Musk make a Nazi salute? Well, not exactly. As far as I can tell, the truth is that he recklessly and repeatedly made a hand gesture that the world’s millions of Nazi sympathizers eagerly misinterpreted as a Nazi salute. He then (the worse part) declined to clarify or apologize in any way, opting instead for laugh emojis.

I hasten to add: just like with Trump’s Charlottesville dogwhistles, I find it ludicrous to imagine that Elon has any secret desire to reopen the gas chambers or whatever—and not only because of Elon’s many pro-Zionist and philosemitic actions, statements, and connections. That isn’t the issue, so don’t pretend I think it is.

Crucially, though, “not being a literal Nazi” isn’t fully exculpatory. I don’t want the overlords of the planet treating these matters as jokes. I want them to feel the crushing weight of history, exactly like I would feel it in their shoes.

Regardless of my distaste for everything that happened to reach this point, Elon is now in a unique position to nudge Trump in the direction of liberality and enlightenment on various issues.  And while I doubt Elon finds time to read Shtetl-Optimized between his CEOing, DOGEing, tweeting, and video game speedruns, I know for certain that there are multiple readers of this blog to whom Elon has listened in the past—and those people are now in a unique position too!

A public “clarification” from Elon—not an apology, not an admission of guilt, but just an acknowledgment that he knows why sleeping dragons like Nazism shouldn’t be poked for shits and giggles, that he’ll try to be careful in the future—would be a non-negligible positive update for me about the future of the world.

I understand exactly why he doesn’t want to do it: because he doesn’t want to grant any legitimacy to what he sees as the biased narrative of a legacy media that despises him. But granting some legitimacy to that narrative is precisely what I, a classically liberal Jewish scientist who bears the battle scars of attempted woke cancellation, am asking him to do. I’m asking him to acknowledge that he’s now by any measure one of the most powerful people on the planet, that with great power comes great responsibility, and that fascism is a well-known failure mode for powerful rightists, just like Communism is a well-known failure mode for leftists. I’m asking for reassurance that he takes that failure mode seriously, just like he correctly takes human extinction and catastrophic AI risk seriously.

Anyway, I figured it was worth a try, given how much I really believe might hinge on how Elon chooses to handle this. I don’t want to be kicking myself, for the rest of my life, that I had a chance to intervene in the critical moment and didn’t.

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Err, happy MLK Day!

This week represents the convergence of so many plotlines that, if it were the season finale of some streaming show, I’d feel like the writers had too many balls in the air. For the benefit of the tiny part of the world that cares what I think, I offer the following comments.

My view of Trump is the same as it’s been for a decade—that he’s a con man, a criminal, and the most dangerous internal threat the US has ever faced in its history. I think Congress and Merrick Garland deserve eternal shame for not moving aggressively to bar Trump from office and then prosecute him for insurrection—that this was a catastrophic failure of our system, one for which we’ll now suffer the consequences. If this time Trump got 52% of some swing state rather than 48%, if the “zeitgeist” or the “vibes” have shifted, if the “Resistance” is so weary that it’s barely bothering to show up, if Bezos and Zuckerberg and Musk and even Sam Altman now find it expedient to placate the tyrant rather than standing up for what previously appeared to be their principles—well, I don’t see how any of that affects how I ought to feel.

All the same, I have no plans to flee the United States or anything, just like I didn’t the last time. I’ll even permit myself pleasure when the crazed strongman takes actions that I happen to agree with (like pushing the tottering Ayatollah regime toward its well-deserved end). And then I’ll vote for Enlightenment values (or the nearest available approximation) in 2026 and 2028, assuming the country survives until then.

The second plotline is the ceasefire in Gaza, and the beginning of the release of the Israeli hostages, in exchange for thousands of Palestinian prisoners. I have all the mixed emotions you might expect. I’m terrified about the precedent this reinforces and about the many mass-murderers it will free—as I was terrified in 2011 by the Gilad Shalit deal, the one that released Sinwar and thereby set the stage for October 7. Certainly World War II didn’t end with the Nazis marching triumphantly around Berlin, guns in the air, and vowing to repeat their conquest of Europe at the earliest opportunity. All the same, it’s not my place to be more Zionist than Netanyahu, or than the vast majority of the Israeli public that supported the deal. I’m obviously thrilled to see the hostages return, and even slightly touched by the ethic that would move heaven and earth to save these specific people, almost every consideration of game theory and utilitarianism be damned. I take solace that we’re not quite returning to the situation of October 6, since Hamas, Hezbollah, and Iran itself have all been severely degraded (and the Assad regime no longer exists). This is no longer 1944, when you can slaughter 1200 Jews without paying any price for it: that was the original promise of the State of Israel. All the same, I fear that bloodshed will continue from here until the Singularity, unless majorities on both sides choose coexistence—partition, the two-state solution, call it whatever you will. And that’s primarily a question of culture, and the education of children.

The third plotline was the end of TikTok, quickly followed by its (temporary?) return on Trump’s order. As far as I can tell, Instagram, Twitter/X, and TikTok have all been net negatives for the world; it would’ve been far better if none of them had been invented. But, OK, our society allows many things that are plausibly net-negative, like sports betting and Cheetos. In this case, however, the US Supreme Court ruled 9-0 (!!) that Congress has a legitimate interest in keeping Chinese Communist Party spyware off 170 million Americans’ phones—and that there’s no First Amendment concern that overrides this security interest, since the TikTok ban isn’t targeting speech on the basis of its content. I found the court’s argument convincing. I hope TikTok goes dark 90 days from now—or, second-best, that it gets sold to some entity that’s merely bad in the normal ways and not a hostile foreign power.

The fourth plotline is the still-ongoing devastation of much of Los Angeles. I heard from friends at Caltech and elsewhere who had to evacuate their homes—but at least they had homes to return to, as those in Altadena and the Palisades didn’t. It’s a sign of the times that even a disaster of this magnitude now brings only partisan bickering: was the cause climate change, reshaping the entire planet in terrifying ways, just like all those experts have been warning for decades? Or was it staggering lack of preparation from the California and LA governments? My own answers to these questions are “yes” and “yes.”

Maybe I’ll briefly highlight the role of the utilitarianism versus deontology debate. According to this article from back in October, widely shared once the fires started, the US Forest Service halted controlled burns in California because it lacked the manpower, but also this:

“I think the Forest Service is worried about the risk of something bad happening [with a prescribed burn]. And they’re willing to trade that risk — which they will be blamed for — for increased risks on wildfires,” Wara said. In the event of a wildfire, “if something bad happens, they’re much less likely to be blamed because they can point the finger at Mother Nature.”

We saw something similar with the refusal to allow challenge trials for the COVID vaccines, which could’ve moved the approval date up by months and saved millions of lives. Humans are really bad at trolley problems, at weighing a concrete, immediate risk against a diffuse future risk that might be orders of magnitude worse. (Come to think of it, Israel’s repeated hostage deals are another example—though that one has the defense that it demonstrates the lengths to which the state will go to protect its people.)

Oh, and on top of all the other plotlines, today—January 20th—is my daughter’s 12th birthday. Happy birthday Lily!!

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Update (Jan. 13): Readers might enjoy the Bankless Podcast, in which I and Justin Drake of the Ethereum engineering team discuss quantum computing and its impact on cryptocurrency. I learned something interesting from Justin—namely that Satoshi has about $90 billion worth of bitcoin that’s never been touched since the cryptocurrency’s earliest days, much of which (added: the early stuff, the stuff not additionally protected by a hash function) would be stealable by anyone who could break elliptic curve cryptography—for example, by using a scalable quantum computer. At what point in time, if any, would this stash acquire the moral or even legal status of (say) gold doubloons just lying on the bottom of the ocean? Arrr, ’tis avast Hilbert space!

Apparently Jensen Huang, the CEO of NVIDIA, opined on an analyst call this week that quantum computing was plausibly still twenty years away from being practical. As a direct result, a bunch of publicly-traded quantum computing companies (including IonQ, Rigetti, and D-Wave) fell 40% or more in value, and even Google/Alphabet stock fell on the news.

So then friends and family attuned to the financial markets started sending me messages asking for my reaction, as the world’s semi-unwilling Quantum Computing Opiner-in-Chief.

My reaction? Mostly just that it felt really weird for all those billions of dollars to change hands, or evaporate, based on what a microchip CEO offhandedly opined about my tiny little field, while I (like much of that field) could’ve remained entirely oblivious to it, were it not for all of their messages!

But was Jensen Huang right in his time estimate? And, relatedly, what is the “correct” valuation of quantum computing companies? Alas, however much more I know about quantum computing than Jensen Huang does, the knowledge does not enable me to answer to either question.

I can, of course, pontificate about the questions, as I can pontificate about anything.

To start with the question of timelines: yes, there’s a lot still to be done, and twenty years might well be correct. But as I’ve pointed out before, within the past year we’ve seen 2-qubit gates with ~99.9% fidelity, which is very near the threshold for practical fault-tolerance. And of course, Google has now demonstrated fault-tolerance that becomes more and more of a win with increasing code size. So no, I can’t confidently rule out commercially useful quantum simulations within the next decade. Like, it sounds fanciful, but then I remember how fanciful it would’ve seemed in 2012 that we’d have conversational AI by 2022. I was alive in 2012! And speaking of which, if you really believe (as many people now do) AI will match or exceed human capabilities in most fields in the next decade, then that will scramble all the other timelines too. And presumably Jensen Huang understands these points as well as anyone.

Now for the valuation question. On the one hand, Shtetl-Optimized readers will know that there’s been plenty of obfuscation and even outright lying, to journalists, the public, and investors, about what quantum computing will be good for and how soon. To whatever extent the previous valuations were based on that lying, a brutal correction was of course in order, regardless of what triggered it.

On the other hand, I can’t say with certainty that high valuations are wrong! After all, even if there’s only a 10% chance that something will produce $100B in value, that would still justify a $10B valuation. It’s a completely different way of thinking than what we’re used to in academia.

For whatever it’s worth, my own family’s money is just sitting in index funds and CDs. I have no quantum computing investments of any kind. I do sometimes accept consulting fees to talk to quantum computing startups and report back my thoughts. When I do, my highest recommendation is: “these people are smart and honest, everything they say about quantum algorithms is correct insofar as I can judge, and I hope they succeed. I wouldn’t invest my own money, but I’m very happy if you or anyone else does.” Meanwhile, my lowest recommendation is: “these people are hypesters and charlatans, and I hope they fail. But even then, I can’t say with confidence that their valuation won’t temporarily skyrocket, in which case investing in them would presumably have been the right call.”

So basically: it’s good that I became an academic rather than an investor.

Having returned from family vacation, I hope to get back to a more regular blogging schedule … let’s see how it goes!

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Yesterday I arrived in Santa Clara for the Q2B (Quantum 2 Business) conference, which starts this morning, and where I’ll be speaking Thursday on “Quantum Algorithms in 2024: How Should We Feel?” and also closing the conference via an Ask-Us-Anything session with John Preskill. (If you’re at Q2B, reader, come and say hi!)

And to coincide with Q2B, yesterday Google’s Quantum group officially announced “Willow,” its new 105-qubit superconducting chip with which it’s demonstrated an error-corrected surface code qubit as well as a new, bigger quantum supremacy experiment based on Random Circuit Sampling. I was lucky to be able to attend Google’s announcement ceremony yesterday afternoon at the Computer History Museum in Mountain View, where friend-of-the-blog-for-decades Dave Bacon and other Google quantum people explained exactly what was done and took questions (the technical level was surprisingly high for this sort of event). I was also lucky to get a personal briefing last week from Google’s Sergio Boixo on what happened.

Meanwhile, yesterday Sundar Pichai tweeted about Willow, and Elon Musk replied “Wow.” It cannot be denied that those are both things that happened.

Anyway, all yesterday, I then read comments on Twitter, Hacker News, etc. complaining that, since there wasn’t yet a post on Shtetl-Optimized, how could anyone possibly know what to think of this?? For 20 years I’ve been trying to teach the world how to fish in Hilbert space, but (sigh) I suppose I’ll just hand out some more fish. So, here are my comments:

1. Yes, this is great. Yes, it’s a real milestone for the field. To be clear: for anyone who’s been following experimental quantum computing these past five years (say, since Google’s original quantum supremacy milestone in 2019), there’s no particular shock here. Since 2019, Google has roughly doubled the number of qubits on its chip and, more importantly, increased the qubits’ coherence time by a factor of 5. Meanwhile, their 2-qubit gate fidelity is now roughly 99.7% (for controlled-Z gates) or 99.85% (for “iswap” gates), compared to ~99.5% in 2019. They then did the more impressive demonstrations that predictably become possible with more and better qubits. And yet, even if the progress is broadly in line with what most of us expected, it’s still of course immensely gratifying to see everything actually work! Huge congratulations to everyone on the Google team for a well-deserved success.
   
2. I already blogged about this!!! Specifically, I blogged about Google’s fault-tolerance milestone when its preprint appeared on the arXiv back in August. To clarify, what we’re all talking about now is the same basic technical advance that Google already reported in August, except now with the PR blitz from Sundar Pichai on down, a Nature paper, an official name for the chip (“Willow”), and a bunch of additional details about it.
   
3. Scientifically, the headline result is that, as they increase the size of their surface code, from 3×3 to 5×5 to 7×7, Google finds that their encoded logical qubit stays alive for longer rather than shorter. So, this is a very important threshold that’s now been crossed. As Dave Bacon put it to me, “eddies are now forming”—or, to switch metaphors, after 30 years we’re now finally tickling the tail of the dragon of quantum fault-tolerance, the dragon that (once fully awoken) will let logical qubits be preserved and acted on for basically arbitrary amounts of time, allowing scalable quantum computation.
   
4. Having said that, Sergio Boixo tells me that Google will only consider itself to have created a “true” fault-tolerant qubit, once it can do fault-tolerant two-qubit gates with an error of ~10^-6 (and thus, on the order of a million fault-tolerant operations before suffering a single error). We’re still some ways from that milestone: after all, in this experiment Google created only a single encoded qubit, and didn’t even try to do encoded operations on it, let alone on multiple encoded qubits. But all in good time. Please don’t ask me to predict how long, though empirically, the time from one major experimental QC milestone to the next now seems to be measured in years, which are longer than weeks but shorter than decades.
   
5. Google has also announced a new quantum supremacy experiment on its 105-qubit chip, based on Random Circuit Sampling with 40 layers of gates. Notably, they say that, if you use the best currently-known simulation algorithms (based on Johnnie Gray’s optimized tensor network contraction), as well as an exascale supercomputer, their new experiment would take ~300 million years to simulate classically if memory is not an issue, or ~10²⁵ years if memory is an issue (note that a mere ~10¹⁰ years have elapsed since the Big Bang). Probably some people have come here expecting me to debunk those numbers, but as far as I know they’re entirely correct, with the caveats stated. Naturally it’s possible that better classical simulation methods will be discovered, but meanwhile the experiments themselves will also rapidly improve.
   
6. Having said that, the biggest caveat to the “10²⁵ years” result is one to which I fear Google drew insufficient attention. Namely, for the exact same reason why (as far as anyone knows) this quantum computation would take ~10²⁵ years for a classical computer to simulate, it would also take ~10²⁵ years for a classical computer to directly verify the quantum computer’s results!! (For example, by computing the “Linear Cross-Entropy” score of the outputs.) For this reason, all validation of Google’s new supremacy experiment is indirect, based on extrapolations from smaller circuits, ones for which a classical computer can feasibly check the results. To be clear, I personally see no reason to doubt those extrapolations. But for anyone who wonders why I’ve been obsessing for years about the need to design efficiently verifiable near-term quantum supremacy experiments: well, this is why! We’re now deeply into the unverifiable regime that I warned about.
   
7. In his remarks yesterday, Google Quantum AI leader Hartmut Neven talked about David Deutsch’s argument, way back in the 1990s, that quantum computers should force us to accept the reality of the Everettian multiverse, since “where else could the computation have happened, if it wasn’t being farmed out to parallel universes?” And naturally there was lots of debate about that on Hacker News and so forth. Let me confine myself here to saying that, in my view, the new experiment doesn’t add anything new to this old debate. It’s yet another confirmation of the predictions of quantum mechanics. What those predictions mean for our understanding of reality can continue to be argued as it’s been since the 1920s.
   
8. Cade Metz did a piece about Google’s announcement for the New York Times. Alas, when Cade reached out to me for comment, I decided that it would be too awkward, after what Cade did to my friend Scott Alexander almost four years ago. I talked to several other journalists, such as Adrian Cho for Science.
   
9. No doubt people will ask me what this means for superconducting qubits versus trapped-ion or neutral-atom or photonic qubits, or for Google versus its many competitors in experimental QC. And, I mean, it’s not bad for Google or for superconducting QC! These past couple years I’d sometimes commented that, since Google’s 2019 announcement of quantum supremacy via superconducting qubits, the trapped-ion and neutral-atom approaches had seemed to be pulling ahead, with spectacular results from Quantinuum (trapped-ion) and QuEra (neutral atoms) among others. One could think of Willow as Google’s reply, putting the ball in competitors’ courts likewise to demonstrate better logical qubit lifetime with increasing code size (or, better yet, full operations on logical qubits exceeding that threshold, without resorting to postselection). The great advantage of trapped-ion qubits continues to be that you can move the qubits around (and also, the two-qubit gate fidelities seem somewhat ahead of superconducting). But to compensate, superconducting qubits have the advantage that the gates are a thousand times faster, which makes feasible to do experiments that require collecting millions of samples.
   
10. Of course the big question, the one on everyone’s lips, was always how quantum computing skeptic Gil Kalai was going to respond. But we need not wonder! On his blog, Gil writes: “We did not study yet these particular claims by Google Quantum AI but my general conclusion apply to them ‘Google Quantum AI’s claims (including published ones) should be approached with caution, particularly those of an extraordinary nature. These claims may stem from significant methodological errors and, as such, may reflect the researchers’ expectations more than objective scientific reality.’ ”  Most of Gil’s post is devoted to re-analyzing data from Google’s 2019 quantum supremacy experiment, which Gil continues to believe can’t possibly have done what was claimed. Gil’s problem is that the 2019 experiment was long ago superseded anyway: besides the new and more inarguable Google result, IBM, Quantinuum, QuEra, and USTC have now all also reported Random Circuit Sampling experiments with good results. I predict that Gil, and others who take it as axiomatic that scalable quantum computing is impossible, will continue to have their work cut out for them in this new world.

Update: Here’s Sabine Hossenfelder’s take. I don’t think she and I disagree about any of the actual facts; she just decided to frame things much more negatively. Ironically, I guess 20 years of covering hyped, dishonestly-presented non-milestones in quantum computing has inclined me to be pretty positive when a group puts in this much work, demonstrates a real milestone, and talks about it without obvious falsehoods!

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Update (Dec. 9): For those who still haven’t gotten enough, check out a 1-hour Zoom panel discussion about quantum algorithms, featuring yours truly along with my distinguished colleagues Eddie Farhi, Aram Harrow, and Andrew Childs, moderated by Barry Sanders, as part of the QTML’2024 conference held in Melbourne (although, it being Thanksgiving week, none of the four panelists were actually there in person). Part of the panel devolves into a long debate between me and Eddie about how interesting quantum algorithms are if they don’t achieve speedups over classical algorithms, and whether some quantum algorithms papers mislead people by not clearly addressing the speedup question (you get one guess as to which side I took). I resolved going in to keep my comments as civil and polite as possible—you can judge for yourself how well I succeeded! Thanks very much to Barry and the other QTML organizers for making this happen.

Do you like watching me spout about AI alignment, watermarking, my time at OpenAI, the P versus NP problem, quantum computing, consciousness, Penrose’s views on physics and uncomputability, university culture, wokeness, free speech, my academic trajectory, and much more, despite my slightly spastic demeanor and my many verbal infelicities? Then holy crap are you in luck today! Here’s 2.5 hours of me talking to former professional poker players (and now wonderful Austin-based friends) Liv Boeree and her husband Igor Kurganov about all of those topics. (Or 1.25 hours if you watch at 2x speed, as I strongly recommend.)

But that’s not all! Here I am talking to Harvard’s Hrvoje Kukina, in a much shorter (45-minute) podcast focused on quantum computing, cosmological bounds on information processing, and the idea of the universe as a computer:

Last but not least, here I am in an hour-long podcast (this one audio-only) with longtime friend Kelly Weinersmith and her co-host Daniel Whiteson, talking about quantum computing.

Enjoy!

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