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What You Shouldn't Know About Quantum Computers
- Etheryte 2y agoAt first I thought that the introduction was hyperbole, but the write-up does actually more or less deliver on it. Written in plain English that anyone can grok, it's a good summary of what quantum is, what it isn't, and why you should care. Definitely recommend reading if, like me, you're not too familiar with the field.
- bigbacaloa 2y ago[dead]
- andrewla 2y agoThe article suffers from skipping between layers of abstraction in an attempt to make a point, and in the end in doing so fails to make the initial point. In the first "Myth" section, that "nobody understands this quantum stuff", the example is used of the transistor. It's true that we have no way of making a classical model of a transistor, and our understanding of how transistors work relies on quantum mechanics. But we did not invent transistors from quantum physics -- we created transistors long before we had an explanation of how they worked, and we have continued to improve and iterate by making advances in material sciences and experimentation, not by applying first principles. Things like blue LEDs were invented by tinkering rather than by solving Lagrangians. The final section was of particular interest to me; Gil Kalai's work on quantum error correction is very interesting to me and I am in the camp that believes that quantum computing is not possible in any useful sense; in particular a quantum computer will not be capable of being significantly more powerful than a classical computer, in the quantum supremacy sense. Here the author reverts to a simplistic argument that "of course" quantum computers are possible, because we can model a quantum computer in a traditional computer. But this sidesteps the main claims, which are whether it is possible to scale error correction to the point where a useful result can be achieved. Even in the domain of NISQ, which is roughly the equivalent of running fluid dynamics simulation in a bathtub, we have yet to produce results showing that we can scale significantly better than a quantum computer.
- neolefty 2y ago> The final section was of particular interest to me; Gil Kalai's work on quantum error correction is very interesting to me and I am in the camp that believes that quantum computing is not possible in any useful sense; in particular a quantum computer will not be capable of being significantly more powerful than a classical computer, in the quantum supremacy sense. My question too. I've had a vague feeling about this for a long time, waving my hands about thermodynamics with "It must get exponentially harder per qbit to eliminate thermal noise by cooling down closer to absolute zero," and I'd really like to get past my hand-waving and see what the dynamics really are.
- sebzim4500 2y ago>It must get exponentially harder per qbit to eliminate thermal noise by cooling down closer to absolute zero Why? Cooling a large object is not exponentially harder than cooling a small object.
- nyrikki 2y agoSurface area is squared, volume is cubed, a larger object has to get hotter to expell the same amount of heat. Per unit of volume, your body produces more heat than the sun, exactly because it is an exponential function.
- MrsPeaches 2y agoChris Ferrie also writes great books about science for babies. https://www.csferrie.com/books https://www.csferrie.com/books https://www.amazon.com/Quantum-Computing-Babies-Baby-University/dp/1492671185 https://www.amazon.com/Quantum-Computing-Babies-Baby-Univers...
- mijoharas 2y agoAt first I saw that this was downvoted and assumed maybe it was a different Chris Ferrie, but from the looks of the blog it's the same person.[0] Maybe other people thought this didn't add much to the discussion, but I found it interesting. [0] I am Chris Ferrie, father of four and happy husband. My day job is academic research where I follow my curiosity through the world of quantum physics. My passion for communicating science has led from the most esoteric topics of mathematical physics to more recently writing children’s books.[1] [1] https://www.csferrie.com/about https://www.csferrie.com/about
- ibcj 2y agoIt is for sure the same Chris Ferrie, it's even in the foreword of the paper: Yet in all that time I never thought to write, much less did I actually write, a pithy book called “What You Shouldn't Know About Quantum Computers.” My colleague Chris Ferrie did. He's the same guy who coauthored the surprise bestseller “Quantum Computing for Babies.” Now he's back, with something for those babies to read when they're slightly older. I enjoy his kids' books and read almost all of them to my kids. They aren't "perfect" (whatever that means for a kids' book), but my kids love them and they start to wrap their brains around otherwise inaccessible topics for their age.
- nathan_compton 2y agoIt might be that I have a Phd in Physics and a long term interest in foundations, but I feel like these books are not very good. Often they emphasize the superficial analogies which physicists use rather than the substantial qualities of the theories. For example, emphasizing spacetime curvature is, in my opinion, kind of a weird way to talk about the actual substance of general relativity (universal coupling of gravity, futility of trying to describe the world with any fixed four dimensional coordinate system). His quantum mechanics book has similar problems. When I try to explain physics to my young child I always try to get at the essence of the ideas, not the superficial pictures.
- AlanYx 2y agoThe linked paper touches on quantum error correction but doesn't explain what the state of the art is. Has any team successfully demonstrated a single usable logical error-corrected qubit yet? I saw this article two months ago https://physicsworld.com/a/why-error-correction-is-quantum-computings-defining-challenge/ https://physicsworld.com/a/why-error-correction-is-quantum-c... discussing the topic, but the writing makes it unclear exactly what was achieved by the various groups. Is recovery from a finite number of errors sufficient to make a usable logical cubit, or is more work still left to be done?
- sebzim4500 2y agoI think this is the SOTA https://s7d9.scene7.com/is/content/quantum/LQ_ErrorCorrectionpdf https://s7d9.scene7.com/is/content/quantum/LQ_ErrorCorrectio... the corrections significantly decrease the error rate but they need to do a lot of preselection, so it isn't quite useful yet.
- AlanYx 2y agoThanks! If I'm reading that correctly, their best error rate with pre and post selection is 0.03%, but they end the paper with the statement "A significant milestone will be to demonstrate a universal family of quantum circuits with logical error rates approaching 10^−8." Seems like we're still six orders of magnitude off.
- sebzim4500 2y agoIIRC We're just over 1 order of magnitude on the physical qbit error before we should see the exponential gains from error correction that theory predicts.
- sampo 2y ago> Researchers like Jaime Sevilla and Jess Riedel support this timeline, publishing a report in late 2020 that claimed a 90% confidence of RSA-2048 being factored before 2060. I am skeptical. 36 years is a long time, but in the past 10 years there hasn't been much progress: year 2001: factorization of 15 (IBM) year 2012: factorization of 21 (University of Bristol) year 2019: factorization of 35 attempt, failed (IBM) https://en.wikipedia.org/wiki/Shor%27s_algorithm#Physical_implementation https://en.wikipedia.org/wiki/Shor%27s_algorithm#Physical_im...
- sebzim4500 2y agoIIRC none of those uses of shor's algorithm were real (well maybe the 2019 one was, but that failed). There's a threshold you need to reach for quantum error correction to work and we are approaching it pretty steadily on a log scale.
- blamestross 2y agolots of functions with horizontal asymptotes look ok on a log scale until they don't.
- JanisErdmanis 2y agoQuantum error correction produces logical qubits that have smaller nonzero error rate. Thus it needs to be applied repeatedly to achieve a necessary error rate to produce meaningful results. For Shor algorithm the error rate needs to decrease exponentially with the number of qubits. Thus even though IBM has a hundred qubits QCs they still only have managed to use five qubits to factorize a number 21.
- sebzim4500 2y ago>For Shor algorithm the error rate needs to decrease exponentially with the number of qubits I have never seen a result like that. Do you have a citation?
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- blamestross 2y agoYou have to get to page 25 before it starts being honest about the fact quantum computing is a con. Important Nuance: the research is real, the science is real, but the narrative being sold about the future of quantum computing to ensnare investors is not a reasonable prediction and is a con.
- bawolff 2y agoTo be fair, that doesn't really distinguish it from the rest of the industry. I would say the same thing about AI and (especially) blockchain.
- nicholast 2y agoThank God someone other than Scott Aaronson is publishing this kind of content. Shetl Optimized is too much of a vanity project to serve as a mainstream resource.
- cwillu 2y agoMeanwhile, the author of the foreward is none other than…
- bee_rider 2y agoIs this really an appropriate use of arxiv? I thought it was for physics preprints. This might be a neat work, but it appears to be a 143 page blog post in a PDF (unless I missed the references section?)
- tom-from-july 2y agoThe text also available as a printed book, so I guess a pdf of it fits any reasonable definition of preprint.
- jessriedel 2y agoThis is in the arXiv section called “physics and society” which is specifically intended for high-quality popular physics book, among other things.
- bee_rider 2y agoGood to know!
- naasking 2y ago> Researchers like Jaime Sevilla and Jess Riedel support this timeline, publishing a report in late 2020 that claimed a 90% confidence of RSA-2048 being factored before 2060. Delusional, IMO. Without a proper understanding of how the non-linearity of the macroscopic world arises from the unitarity of QM, any scaling projections are wishful thinking. We just don't have a good enough understanding of the measurement problem and decoherence to make such projections.
- renonce 2y ago> For example, would quantum computers work by trying all possible answers in parallel? Sorry, no, that's too good to be true: Quantum computers work by choreographing a pattern of interference, where the contributions to the amplitude of each wrong answer cancel each other out, while the contributions to the right answer's amplitude reinforce each other. Only for special problems, as it turns out, do we know how to choreograph such an interference pattern to deliver a huge speedup over the best known classical algorithms. This, in turn, is why we don't expect quantum computers ever to replace classical computers, but “merely” to complement them, accelerating specific tasks like quantum simulation and codebreaking. I'm not sure about the field of physics but in deep learning there are hundreds of papers published every day while no more than a percent of them tries to make the paper less mythical and instead they keep inventing buzzwords and claiming positive results to make them even more mythical
- _heimdall 2y agoThat phenomenon isn't specific to physics or deep learning. Academic papers are more and more of a joke these days. Sure there's plenty of good work being done too, but its be buried in a pile of poorly done research and deceiving statistics that are written only to chase funding and/or recognition for the author (promotions, graduation, jobs, etc).
- nyrikki 2y agoNeither NP or NP∩coNP, are contained in BQP, for those that want a complexity theory version of the above. BQP: Bounded-Error Quantum Polynomial-Time, bounded by a max error of 1:3 BQP is the complexity class thought to contain problems with practical solutions for quantum computers. IIRC the main limit being the transition amplitudes are subject to the Church–Turing thesis and must be computable functions. Hopefully useful buzzwords for those who want to dig deeper.
- renonce 2y agoYeah these buzzwords are very useful pointers to wikipedia pages with details on what problems are in which complexity class and what are not. I'm more familiar with cryptography so the most famous problem in BQP for me is discrete logarithm. Once you have this primitive, the following things are very clear: 1. How Shor's algorithm for factorization works: it consists of a classical algorithm that reduces factorization to calculation of group order of an element (which is a special case of discrete logarithm), then uses a quantum computer to solve the group order problem. This breaks RSA. 2. Breaking elliptic cryptography: Modern elliptic cryptography constructs an elliptic curve (in the form of y^2=x^3+Ax+B) and defines multiplication on top of the points on the curve. It turns out that multiplication is very easy but discrete logarithm is hard and that hardness is used to prove that Diffie-Hellman key exchange is hard to break, but what if it's not? Moreover, elliptic curves usually only have 256~512 bits since it's sufficient to guarantee security in the classical case, compared to RSA with 2048~4096 bits. While it's harder to break elliptic curves using classical methods, it turns out to be even easier for quantum computers. What quantum computers is NOT is a parallel computer with 2^n threads running in parallel that would collapse to the thread that gives the correct results. This would imply BQP=NP which is not known to be true and however many qubits we build it won't be any more likely to become true.
- Anotheroneagain 2y agoIt will never work, the physics is completely wrong.
- Suppafly 2y ago>It will never work, the physics is completely wrong. How so?
- pbhjpbhj 2y agoFwiw, there's an event I was just looking at from UCL on the "Future of Quantum Computing" (online webinar), https://www.eventbrite.co.uk/e/the-future-of-quantum-computing-tickets-909692955117 https://www.eventbrite.co.uk/e/the-future-of-quantum-computi... It's a panel headed by Prof Al-Khalili. I suspect it will be a beginner level presentation. >Professor Jim Al-Khalili CBE FRS is a theoretical physicist at the University of Surrey where he holds a Distinguished Chair in physics and leads the Quantum Foundations and Technologies Research Group in the School of Mathematics and Physics. As well as his academic work he is a well-known popular science author and broadcaster on BBC radio and television. No affiliation, just may be of interest.
- DasCorCor 2y agoIt's a better use of your time just learning quantum mechanics than reading this book. Even if you give up, you'll at least learn some linear algebra along the way.
- juped 2y ago@dang why was this flagged off the front page? Isn't this a blatantly obvious abuse of the flagging system?