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Unrelated Bulletins: See here for an extended interview with me in The Texas Orator, protecting the same old stuff (quantum computing, complexity idea, AI security). And see here for a podcast with me and Spencer Greenberg a few related mixture of matters.


A pair weeks in the past, I helped arrange UmeshFest: Don’t Miss This Flight, a workshop at UC Berkeley’s Simons Institute to have fun the two6th birthday of my former PhD adviser Umesh Vazirani. Peter Shor, John Preskill, Manuel Blum, Madhu Sudan, Sanjeev Arora, and dozens of different luminaries of quantum and classical computation had been readily available to assist inform the story of quantum computing idea and Umesh’s central position in it. There was additionally fixed roasting of Umesh—of his life classes from the squash court docket, his last-minute organizational modifications and cellphone calls at random hours. I used to be delighted to search out that my outdated coinage of “Umeshisms” was merely commonplace utilization among the many attendees.


At Berkeley, many issues had been as I remembered them—my favourite Thai eatery, the bubble tea, the Campanile—however not the whole lot was the identical. Right here I’m in entrance of Berkeley’s Gaza encampment, a.ok.a. its “Anti Zionism Zone” or what was previously Sproul Plaza (zoom into the chalk):

I felt a must stroll by way of the Anti Zionism Zone day after day (albeit unassumingly, neither draped in an Israeli flag nor trying to begin an argument with anybody), for more-or-less the identical the explanation why the US usually sends plane carriers by way of the Strait of Taiwan.


Again within the extra sheltered surroundings of the Simons Institute, it was nice to be amongst pals, a few of whom I hadn’t seen since earlier than Covid. Andris Ambainis and I labored collectively for a bit on an open downside in quantum question complexity, for outdated occasions’ sake (we haven’t solved it but).

After which there have been talks! I assumed I’d share my very own discuss, which was entitled The Story of BQP (Bounded-Error Quantum Polynomial-Time). Here are the PowerPoint slides, however I’ll additionally share screen-grabs for these of you who consistently complain that you could’t open PPTX recordsdata.

I used to be significantly pleased with the design of my title slide:

Shifting on:

The category BQP/qpoly, I ought to clarify, is all about an advisor who’s all-wise and completely benevolent, however who doesn’t have a whole lot of time to satisfy together with his college students, so he merely doles out the identical generic recommendation to all of them, no matter their thesis downside x.

I then displayed my infamous “Umeshisms” blog post from 2005—one of many first posts within the historical past of this weblog:

As I defined, now that I hang around with the rationalist and AI security communities, that are additionally headquartered in Berkeley, I’ve discovered that my “Umeshisms” submit by some means took on a lifetime of its personal. As soon as, when eating at one of many rationalists’ polyamorous Berkeley group homes, I stated this has been pretty however I’ll now want to depart, to go to my PhD former adviser Umesh Vazirani. “You imply the Umesh?!” the rationalists excitedly exclaimed. “Of Umeshisms? In case you’ve by no means missed a flight?”

However shifting on:

(Word that by “QBPP,” Bethiaume and Brassard meant what we now name BQP.)

Feynman and Deutsch requested precisely the fitting query—does simulating quantum mechanics on a classical pc inherently produce an exponential slowdown, or not?—however they lacked many of the instruments to begin formally investigating the query. An element-of-two quantum speedup for the XOR operate may very well be dismissed as unimpressive, whereas a a lot better quantum speedup for the “fixed vs. balanced” downside may very well be dismissed as a win in opposition to solely deterministic classical algorithms, fairly than randomized algorithms. Deutsch-Jozsa could have been the primary time that an obvious quantum speedup faltered in an sincere comparability in opposition to classical algorithms. It definitely wasn’t the final!

Ah, however that is the place Bernstein and Vazirani enter the scene.

Bernstein and Vazirani didn’t merely outline BQP, which stays the central object of examine in quantum complexity idea. In addition they established its most elementary properties:

And, at the least within the black-box mannequin, Bernstein and Vazirani gave the primary spectacular quantum speedup for a classical downside that survived in a good comparability in opposition to one of the best classical algorithm:

The Recursive Bernstein-Vazirani downside, additionally known as Recursive Fourier Sampling, is constructed as a “tree” of cases of the Bernstein-Vazirani downside, the place to question the Boolean operate at any given stage, you’ll want to remedy a Bernstein-Vazirani downside for a Boolean operate on the stage under it, after which run the key string s by way of a hard and fast Boolean operate g. For extra, see my outdated paper Quantum Lower Bound for Recursive Fourier Sampling.

Every Bernstein-Vazirani occasion has classical question complexity n and quantum question complexity 1. So, if the tree of cases has depth d, then general the classical question complexity is nd, whereas the quantum question complexity is barely 2d. The place did the two come from? From the necessity to uncompute the key strings s at every stage, to allow quantum interference on the subsequent stage up—thereby forcing us to run the algorithm twice. A key perception.

The Recursive Fourier Sampling separation set the stage for Simon’s algorithm, which gave a extra spectacular speedup within the black-box mannequin, and thence for the well-known Shor’s algorithm for factoring and discrete log:

However Umesh wasn’t achieved establishing probably the most elementary properties of BQP! There’s additionally the seminal 1994 paper by Bennett, Bernstein, Brassard, and Vazirani:

In gentle of the BV and BBBV papers, let’s see how BQP appears to suit with classical complexity courses—an understanding that’s remained largely steady for the previous 30 years:

We are able to state a big fraction of the analysis agenda of the entire subject, to this present day, as questions on BQP:

I gained’t have time to debate all of those questions, however let me at the least drill down on the primary few.

Many individuals hoped the checklist of identified issues in BQP would now be longer than it’s. So it goes: we don’t resolve the reality, we solely uncover it.

As a 17-year-old simply studying about quantum computing in 1998 by studying the Bernstein-Vazirani paper, I used to be thrilled after I managed to enhance their containment BQP ⊆ P#P to BQP ⊆ PP. I assumed that may be my huge debut in quantum complexity idea. I used to be then crushed after I discovered that Adleman, DeMarrais, and Huang had proved the identical factor a yr prior. OK, however at the least it wasn’t, like, 50 years prior! Perhaps if I saved at it, I’d attain the frontier quickly sufficient.

Umesh, from the very starting, raised the profound query of BQP’s relation to the polynomial hierarchy. Might we at the least assemble an oracle relative to which BQP⊄PH—or, intently associated, relative to which P=NP≠BQP? Recursive Fourier Sampling was a already candidate for such a separation. I spent months making an attempt to show that candidate wasn’t in PH, however failed. That led me ultimately to suggest a really totally different downside, Forrelation, which appeared like a stronger candidate, though I couldn’t show that both. Lastly, in 2018, after 4 years of effort, Ran Raz and Avishay Tal proved that my Forrelation downside was not in PH, thereby resolving Umesh’s query after 1 / 4 century.

We now know three alternative ways by which a quantum pc cannot merely remedy any BQP downside effectively, however show its reply to a classical skeptic through an interactive protocol! Utilizing quantum communication, utilizing two entangled (however non-communicating) quantum computer systems, or utilizing cryptography (this final a breakthrough of Umesh’s PhD pupil Urmila Mahadev). It stays an excellent open downside, first posed to my data by Daniel Gottesman, whether or not one can do it with none of these items.

To see lots of the benefits of quantum computation over classical, we’ve discovered that we have to broaden our imaginative and prescient past BQP (which is a category of languages), to promise issues (like estimating the expectation values of observables), sampling issues (like BosonSampling and Random Circuit Sampling), and relational issues (just like the Yamakawa-Zhandry problem, topic of a latest breakthrough). It’s conceivable that quantum benefit may stay for such issues even when it turned out that P=BQP.

A wider query is whether or not BQP captures all languages that may be effectively determined utilizing “cheap bodily assets.” What about chiral quantum subject theories, just like the Normal Mannequin of elementary particles? What about quantum theories of gravity? Good questions!

Because it was Passover through the discuss, I actually stated “Dayenu” to Umesh: “in the event you had solely given us BQP, that may’ve been sufficient! however you didn’t, you gave us a lot extra!”

Comfortable birthday Umesh!! We look ahead to celebrating once more on all of your subsequent power-of-2 birthdays.

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