Top all-time · #2037
Craig Gidney
Personal blog · algassert.comsciencemath
Craig Gidney writes Algorithmic Assertions, a personal blog at algassert.com about science and math. Since 2017, 37 different Hacker News readers have linked it 77 times, most often “Why haven't quantum computers factored 21 yet?”. It is still writing: the last post was on 2026-10-22, and you can follow it by feed.
Too few sampled comments (27) to say how often HN recommends it.?
Why HN links it
HN readers link Gidney's blog mainly for its post on why factoring 21 on a quantum computer is much harder than factoring 15, and for Quirk, the in-browser quantum circuit simulator hosted there.
Summarised by Claude from a sample of 27 HN comments that link here. Each comment is marked recommending, critical or neutral (citing without judging); how this works.
Craig Gidney's computer science blog
Most-linked posts?
- Why haven't quantum computers factored 21 yet?10 people · 13 mentions · first linked 2025-08-31
- Quirk: Quantum Circuit Simulator7 people · 8 mentions · first linked 2018-07-22
- Quirk – A drag-and-drop quantum circuit simulator that runs in your browser3 people · 4 mentions · first linked 2017-08-11
- Factoring the largest number ever with a quantum computer3 people · 3 mentions · first linked 2020-04-02
- You versus Bell Tests and the No Communication Theorem (2015)2 people · 3 mentions · first linked 2022-10-04
- Quantum Error Correction Goes FOOM2 people · 3 mentions · first linked 2025-12-25
- An Explanation of Shor's Quantum Factoring Algorithm2 people · 2 mentions · first linked 2017-08-28
- The predictable failure of the QDay Prize2 people · 2 mentions · first linked 2026-04-26
- The French Have the Quantum Circuits2 people · 2 mentions · first linked 2026-06-02
- Quantum Circuit Simulator: Quirk2 people · 2 mentions · first linked 2018-11-21
What Hacker News says?
Strilanc, who works on the google quantum team, has a simulator here: https://algassert.com/quirk
Comment linking /quirk
on “Ask HN: What scientific phenomenon do you wish someone would explain better?”Submitted Why haven't quantum computers factored 21 yet? as a story · 340 points
HN discussionI know what you're talking about, but I think you happened to pick a bad example to pick on here. This wind tunnel analogy resembles a common criticism of the prior experiments that were done by Google and others over the last few years. Those experiments ran highly unstructured, arbitrary circuits that don't compute anything useful. They hardly resembled the kind of results that you would expect from a general purp…
Comment linking /post/2500
on “Willow quantum chip demonstrates verifiable quantum advantage on hardware”Submitted Bugs from the Future: Hadamard Coins and Implicit Measurement as a story · 63 points
HN discussionJust gonna leave this here: Why haven't quantum computers factored 21 yet? https://algassert.com/post/2500
Comment linking /post/2500
on “Willow quantum chip demonstrates verifiable quantum advantage on hardware”Submitted You can't test if quantum uses complex numbers as a story · 61 points
HN discussionYes. Here's an online drag and drop simulator: https://algassert.com/quirk Until recently [1], classical simulation was faster/cheaper/more-accurate than any existing quantum hardware. But the hardware has been improving and all classical simulation of quantum computation takes exponential time with respect to some important property such as the number of qubits, the depth of the computation, the number of non-trivi…
Comment linking /quirk
on “Amazon Braket – Get Started with Quantum Computing”Submitted An Explanation of Shor's Quantum Factoring Algorithm as a story · 61 points
HN discussionQC simulation (qubits only, no dedicated vis) is much easier. You just make a vector, and multiply it by matrices. Here for Python interface I like QuTiP even more than Qiskit (though, I might be biased, as I contributed to the former). For interactive, IBM Quantum Computing Experience and Quirk ( https://algassert.com/quirk ). As "easier" I mean that 2^10 is still managable, as you can use dense vectors and operato…
Comment linking /quirk
on “Gravity is a double copy of other forces”Submitted Shor's Quantum Factoring Algorithm (2017) as a story · 53 points
HN discussionThe QFT is the Cooley-Tukey FFT algorithm [1] expressed as a tensor network [2]. Cooley-Tukey has two main steps that are repeated recursively: bulk replacing a,b with a+b,a-b along bit boundaries, and applying twiddle factors. The bit-boundary a,b->a+b,a-b part becomes a Hadamard gate and the bit twiddling becomes a set of phase gates. Also there's some re-ordering but that's not the meat. The actual quantum circui…
Comment linking /2016/06/14/qft-by-multiply.html
on “Shor, I’ll do it (2007)”Submitted Computing Fractional Fourier Transforms as a story · 51 points
HN discussionYour code is not properly enforcing the communication constraint between the two players. Try to enter code that passes the bell tester widget included in this blog post and you'll find the problem: https://algassert.com/quantum/2015/10/11/Bell-Tests-vs-No-Co...
Comment linking /quantum/2015/10/11/Bell-Tests-vs-No-Communication.html
on “Nobel Prize in Physics Awarded to Alain Aspect, John Clauser and Anton Zeilinger”Submitted Quirk: Quantum Circuit Simulator as a story · 41 points
HN discussionQuirk: https://algassert.com/quirk - Drag-and-drop what-you-see-is-what-you-get UI instead of script-based. Smooths out the learning curve. - Supports putting state displays in the middle of the circuit, so you can directly view normally-inaccessible information instead of inferring it from experience or algebra. - Fast. It updates all displays interactively, as you edit the circuit. Very easy to experiment, e.g. ju…
Comment linking /quirk
on “Getting Started with Quantum Computing in Python”Submitted The Frauchiger-Renner Paradox Is a Sleeping Beauty Problem as a story · 4 points
HN discussion> An interesting project would be to create a single environment which can be used for both education and research. This does happen sometimes, by accident. I wrote Quirk [1] as an educational tool for learning about quantum circuits. But then I started finding it useful as a tool for optimizing circuits to use fewer gates, just by dragging things around and seeing what happens (there's still enough low-hanging frui…
Comment linking /quirk
on “Magic Paper”Submitted You versus Bell Tests and the No Communication Theorem (2015) as a story · 3 points
HN discussionI just recently played with an online quantum computer simulator to get a better understanding of how quantum fourier transform works, it's a lot of fun: https://algassert.com/quirk
Comment linking /quirk
on “Elliptic Curve Cryptography: A Basic Introduction”Submitted Factoring the largest number ever with a quantum computer as a story · 3 points
HN discussionHi, I personally have a similar background to you, with ~15 years of software engineering. I second some of the comments: I really started understanding quantum computing much better when I sat down and worked through the problems of the Nielsen and Chuang book ("Mike & Ike") - the first 4 chapters should give you a solid start. It starts from theoretical base and does cover some of the applications of QCs as well,…
Comment linking /quirk
on “Ask HN: Resources to start learning about quantum computing?”Submitted Quirk – A drag-and-drop quantum circuit simulator that runs in your browser as a story · 3 points
HN discussionQuantum teleportation is in fact a quantum generalization of the classical one time pad. And it does form a secure channel, but with the same caveats as in the classical case (e.g. it's not authenticated). Many quantum things have classical analogues. For example, there is classical no cloning [1] and classical delayed erasure [2], though people will debate if these classical versions have really captured the "essen…
Comment linking /post/1720
on “Delft scientists make first “on demand” entanglement link”Submitted Visualizing 2-Qubit Entanglement as a story · 3 points
HN discussionI don't know the exact answer, but the lower bound I computed had over a quarter million digits . See [1] 1: http://algassert.com/puzzle/2014/07/08/Boxing-Megaspheres.ht...
Comment linking /puzzle/2014/07/08/Boxing-Megaspheres.html
on “Executing gradient descent on the earth”Submitted Quantum Error Correction Goes FOOM as a story · 2 points
HN discussionThat's not correct; you cannot use a double-slit test to check for entanglement. Running a photon through a double-slit setup always just produces a single dot, not a any sort of pattern. To get a pattern, you need to run a bunch of photons through it and see if a fringe pattern appears [1]. (BTW, you never get a two-line pattern in a decent setup. This is an incredibly common mistake, but it's simply wrong. The int…
Comment linking /quantum/2016/01/07/Delayed-Choice-Quantum-Erasure.html
on “A rudimentary quantum network link between Dutch cities”Submitted Quantum Circuit Simulator: Quirk as a story · 2 points
HN discussionIf it were just a probability paradox, Bell's theorem wouldn't be a thing. You can win certain coordination games more often using quantum correlations than is possible with classical correlations. That being said, I have complained about specific papers and articles making this exact mistake. For example, explanations of delayed choice eraser experiments are almost always confusing correlation or post-selection for…
Comment linking /post/1720
on “Oil droplets guided by pilot waves do not give rise to double-slit interference”Submitted Why Will Quantum Computers Be Slow? as a story · 2 points
HN discussion
Linked by the same people?
- A
Abhishaike Mahajanowlposting.com - R
RÉSONAANCESresonaances.blogspot.com - L
Lance Fortnow and Bill Gasarchblog.computationalcomplexity.org - S
State of the Smeritysmerity.com - J
John Carlos Baezjohncarlosbaez.wordpress.com - S
Scott Aaronsonscottaaronson.blog - T
Timothy Gowersgowers.wordpress.com - S
Sabine Hossenfelderbackreaction.blogspot.com