Craig Gidney
@craiggidney.bsky.social
Research scientist on Google's quantum team, working on reducing the cost of quantum error correction.
Useful tools I've made:
- Quirk: https://algassert.com/quirk
- Stim: https://github.com/quantumlib/stim
- Crumble: https://algassert.com/crumble
Useful tools I've made:
- Quirk: https://algassert.com/quirk
- Stim: https://github.com/quantumlib/stim
- Crumble: https://algassert.com/crumble
This new quanta article (www.quantamagazine.org/physicists-t... arxiv.org/pdf/0810.1923) claims my blog post on spoofing real-only tests is a new discovery. I'm flattered... but since writing the post I learned McKague et al knew it in 2008 (see section II.C of arxiv.org/pdf/0810.1923).
Physicists Take the Imaginary Numbers Out of Quantum Mechanics | Quanta Magazine
Quantum mechanics has at last been formulated exclusively with real numbers, bringing a mathematical puzzle at the heart of the theory into a new era of inquiry.
www.quantamagazine.org
November 8, 2025 at 6:55 AM
This new quanta article (www.quantamagazine.org/physicists-t... arxiv.org/pdf/0810.1923) claims my blog post on spoofing real-only tests is a new discovery. I'm flattered... but since writing the post I learned McKague et al knew it in 2008 (see section II.C of arxiv.org/pdf/0810.1923).
OpenSSH added a "your session isn't secure against store-now-decrypt-later quantum attacks" warning: www.openssh.org/pq.html
I love it. Waiting until the problem is manifest is a great way to have waited too long. Fix shit now.
I love it. Waiting until the problem is manifest is a great way to have waited too long. Fix shit now.
OpenSSH: Post-Quantum Cryptography
OpenSSH post quantum cryptography
www.openssh.org
November 7, 2025 at 11:15 PM
OpenSSH added a "your session isn't secure against store-now-decrypt-later quantum attacks" warning: www.openssh.org/pq.html
I love it. Waiting until the problem is manifest is a great way to have waited too long. Fix shit now.
I love it. Waiting until the problem is manifest is a great way to have waited too long. Fix shit now.
Stim has received its first AI slop pull request: github.com/quantumlib/S...
(It's actually not the first AI generated pull request that's been submitted, but the other one was actually a correct bug fix thus not slop.)
(It's actually not the first AI generated pull request that's been submitted, but the other one was actually a correct bug fix thus not slop.)
October 27, 2025 at 5:08 PM
Stim has received its first AI slop pull request: github.com/quantumlib/S...
(It's actually not the first AI generated pull request that's been submitted, but the other one was actually a correct bug fix thus not slop.)
(It's actually not the first AI generated pull request that's been submitted, but the other one was actually a correct bug fix thus not slop.)
Here's the recording of my talk at the Simons institute quantum industry day: www.youtube.com/live/SULOaOQ...
And the slides: docs.google.com/presentation...
I was also on the panel discussion at the end of the day (also in the linked playlist).
And the slides: docs.google.com/presentation...
I was also on the panel discussion at the end of the day (also in the linked playlist).
Optimizing the Annoying Stuff: Reducing Costs Obscured by the Abstract Circuit Model
YouTube video by Simons Institute for the Theory of Computing
www.youtube.com
October 17, 2025 at 6:27 AM
Here's the recording of my talk at the Simons institute quantum industry day: www.youtube.com/live/SULOaOQ...
And the slides: docs.google.com/presentation...
I was also on the panel discussion at the end of the day (also in the linked playlist).
And the slides: docs.google.com/presentation...
I was also on the panel discussion at the end of the day (also in the linked playlist).
My contribution to arxiv.org/abs/2510.10967 is a space-efficient strategy for modular division under superposition. It's based on recording the branches taken during a GCD, to decompose a number+modulus into their "dialog representation", which unlike binary is entirely linear.
October 14, 2025 at 4:40 AM
My contribution to arxiv.org/abs/2510.10967 is a space-efficient strategy for modular division under superposition. It's based on recording the branches taken during a GCD, to decompose a number+modulus into their "dialog representation", which unlike binary is entirely linear.
I'll be speaking at the Simons Institute this Thursday: simons.berkeley.edu/events/quant... (Hopefully my flight doesn't get cancelled this time.)
Talk Title: Optimizing the Annoying Stuff: Reducing Costs Obscured by the Abstract Circuit Model
Abstract: attached as image
Talk Title: Optimizing the Annoying Stuff: Reducing Costs Obscured by the Abstract Circuit Model
Abstract: attached as image
October 13, 2025 at 10:23 PM
I'll be speaking at the Simons Institute this Thursday: simons.berkeley.edu/events/quant... (Hopefully my flight doesn't get cancelled this time.)
Talk Title: Optimizing the Annoying Stuff: Reducing Costs Obscured by the Abstract Circuit Model
Abstract: attached as image
Talk Title: Optimizing the Annoying Stuff: Reducing Costs Obscured by the Abstract Circuit Model
Abstract: attached as image
For their student researcher-ship, Satoshi Yoshida extended our work on dynamic surface code circuits to color codes. They built and simulated iswap color code circuits and wiggling color code circuits. The iswap circuits use fewer entangling layers than CX circuits!
arxiv.org/abs/2510.00370
arxiv.org/abs/2510.00370
Low Depth Color Code Circuits with CXSWAP gate
We present two new types of syndrome extraction circuits for the color code. Our first construction, which after [M. McEwen, D. Bacon, and C. Gidney, Quantum 7, 1172 (2023)] we call the semi-wiggling ...
arxiv.org
October 2, 2025 at 3:09 AM
For their student researcher-ship, Satoshi Yoshida extended our work on dynamic surface code circuits to color codes. They built and simulated iswap color code circuits and wiggling color code circuits. The iswap circuits use fewer entangling layers than CX circuits!
arxiv.org/abs/2510.00370
arxiv.org/abs/2510.00370
This HSBC paper reminds me of www.youtube.com/watch?v=EbfJ...
In the clip at 38:10, a speaker relays Pons being told a light water control had excess heat. Instead of "oh no", Pons replies "That's the most exciting thing, we see it too!". The audience bursts into laughter at the clear self-fooling.
In the clip at 38:10, a speaker relays Pons being told a light water control had excess heat. Instead of "oh no", Pons replies "That's the most exciting thing, we see it too!". The audience bursts into laughter at the clear self-fooling.
October 1, 2025 at 5:42 PM
This HSBC paper reminds me of www.youtube.com/watch?v=EbfJ...
In the clip at 38:10, a speaker relays Pons being told a light water control had excess heat. Instead of "oh no", Pons replies "That's the most exciting thing, we see it too!". The audience bursts into laughter at the clear self-fooling.
In the clip at 38:10, a speaker relays Pons being told a light water control had excess heat. Instead of "oh no", Pons replies "That's the most exciting thing, we see it too!". The audience bursts into laughter at the clear self-fooling.
Blog post: "Actually, you can't test if quantum uses complex numbers" algassert.com/post/2501
I doom the concept of that 2021 Nature paper by showing how to compile any distributed quantum protocol into real-only gates while preserving locality.
I doom the concept of that 2021 Nature paper by showing how to compile any distributed quantum protocol into real-only gates while preserving locality.
September 15, 2025 at 4:20 AM
Blog post: "Actually, you can't test if quantum uses complex numbers" algassert.com/post/2501
I doom the concept of that 2021 Nature paper by showing how to compile any distributed quantum protocol into real-only gates while preserving locality.
I doom the concept of that 2021 Nature paper by showing how to compile any distributed quantum protocol into real-only gates while preserving locality.
Remember the Nature paper constructing an experiment to distinguish real-number-only QM from complex-number-using QM?
In the supplementary material, they admit it doesn't work if pre-shared entanglement is present. Which there's no way to test for. So the experiment doesn't actually do the thing.
In the supplementary material, they admit it doesn't work if pre-shared entanglement is present. Which there's no way to test for. So the experiment doesn't actually do the thing.
September 12, 2025 at 1:42 AM
Remember the Nature paper constructing an experiment to distinguish real-number-only QM from complex-number-using QM?
In the supplementary material, they admit it doesn't work if pre-shared entanglement is present. Which there's no way to test for. So the experiment doesn't actually do the thing.
In the supplementary material, they admit it doesn't work if pre-shared entanglement is present. Which there's no way to test for. So the experiment doesn't actually do the thing.
Apparently September 5th is "cultivate with fold-transversal S" day:
arxiv.org/abs/2509.05232
arxiv.org/abs/2502.017...
arxiv.org/abs/2509.05212
arxiv.org/abs/2509.05232
arxiv.org/abs/2502.017...
arxiv.org/abs/2509.05212
September 8, 2025 at 9:53 PM
Apparently September 5th is "cultivate with fold-transversal S" day:
arxiv.org/abs/2509.05232
arxiv.org/abs/2502.017...
arxiv.org/abs/2509.05212
arxiv.org/abs/2509.05232
arxiv.org/abs/2502.017...
arxiv.org/abs/2509.05212
I used to write TODOs in my code. Now I write DIDNTDOs. Because let's be honest... these are apologies, not promises.
// DIDNTDO: handle negative values
// DIDNTDO: handle negative values
September 3, 2025 at 2:33 AM
I used to write TODOs in my code. Now I write DIDNTDOs. Because let's be honest... these are apologies, not promises.
// DIDNTDO: handle negative values
// DIDNTDO: handle negative values
Been awhile since I wrote a blog post.
Anyways... why haven't quantum computers factored 21 yet?
algassert.com/post/2500
Anyways... why haven't quantum computers factored 21 yet?
algassert.com/post/2500
Why haven't quantum computers factored 21 yet?
Craig Gidney's computer science blog
algassert.com
August 31, 2025 at 12:21 AM
Been awhile since I wrote a blog post.
Anyways... why haven't quantum computers factored 21 yet?
algassert.com/post/2500
Anyways... why haven't quantum computers factored 21 yet?
algassert.com/post/2500
Was wondering why my code suddenly got HALF AS FAST.
I traced it to extra error info.. in a switch default that never even runs! I think the compiler was unconditionally constructing a location for the temporary string or something?? 🤮
Clear example why people trust C more than C++.
I traced it to extra error info.. in a switch default that never even runs! I think the compiler was unconditionally constructing a location for the temporary string or something?? 🤮
Clear example why people trust C more than C++.
August 27, 2025 at 1:15 AM
Was wondering why my code suddenly got HALF AS FAST.
I traced it to extra error info.. in a switch default that never even runs! I think the compiler was unconditionally constructing a location for the temporary string or something?? 🤮
Clear example why people trust C more than C++.
I traced it to extra error info.. in a switch default that never even runs! I think the compiler was unconditionally constructing a location for the temporary string or something?? 🤮
Clear example why people trust C more than C++.
Been experimenting with generating circuit-specialized simulation code, to reduce branch mispredictions.
Generating a 1M line NASM file has gone far better than a 1M line C++ file. nasm finishes in a few seconds on inputs of that size (gcc was taking hours). But still unnecessarily slow...
Generating a 1M line NASM file has gone far better than a 1M line C++ file. nasm finishes in a few seconds on inputs of that size (gcc was taking hours). But still unnecessarily slow...
August 23, 2025 at 9:15 PM
Been experimenting with generating circuit-specialized simulation code, to reduce branch mispredictions.
Generating a 1M line NASM file has gone far better than a 1M line C++ file. nasm finishes in a few seconds on inputs of that size (gcc was taking hours). But still unnecessarily slow...
Generating a 1M line NASM file has gone far better than a 1M line C++ file. nasm finishes in a few seconds on inputs of that size (gcc was taking hours). But still unnecessarily slow...
The recording of my talk on denser planar storage, at QEC2025: yale.hosted.panopto.com/Panopto/Page...
And the slides: docs.google.com/presentation...
And the slides: docs.google.com/presentation...
Yoked surface codes - QEC2025
Hello everyone my name is Craig and I'm going to be talking about techniques for reducing the spatial overhead of surface codes without asking for anything more from hardware Yoked surface codes Craig...
docs.google.com
August 15, 2025 at 1:44 AM
The recording of my talk on denser planar storage, at QEC2025: yale.hosted.panopto.com/Panopto/Page...
And the slides: docs.google.com/presentation...
And the slides: docs.google.com/presentation...
An identity that'd be great... if it did 2 things instead of 3.
If one of the two CCZs on the right wasn't there, it'd yield an n-qubit incrementer with ̶4̶n̶→3n T gates.
If the CCCZ wasn't there, it'd yield n single-shared-control Toffolis to be done with ̶4̶n̶→3n T gates.
If one of the two CCZs on the right wasn't there, it'd yield an n-qubit incrementer with ̶4̶n̶→3n T gates.
If the CCCZ wasn't there, it'd yield n single-shared-control Toffolis to be done with ̶4̶n̶→3n T gates.
August 2, 2025 at 1:51 PM
An identity that'd be great... if it did 2 things instead of 3.
If one of the two CCZs on the right wasn't there, it'd yield an n-qubit incrementer with ̶4̶n̶→3n T gates.
If the CCCZ wasn't there, it'd yield n single-shared-control Toffolis to be done with ̶4̶n̶→3n T gates.
If one of the two CCZs on the right wasn't there, it'd yield an n-qubit incrementer with ̶4̶n̶→3n T gates.
If the CCCZ wasn't there, it'd yield n single-shared-control Toffolis to be done with ̶4̶n̶→3n T gates.
The first novel thing I did in quantum computing was find a way add +1 to a register using O(1) space and O(n) gates. For 10 years I've wanted to know how to generalize that from x+=1 to x+=C... and I finally figured out a way to do it!
arxiv.org/abs/2507.23079
arxiv.org/abs/2507.23079
August 1, 2025 at 3:03 AM
The first novel thing I did in quantum computing was find a way add +1 to a register using O(1) space and O(n) gates. For 10 years I've wanted to know how to generalize that from x+=1 to x+=C... and I finally figured out a way to do it!
arxiv.org/abs/2507.23079
arxiv.org/abs/2507.23079
Chevignard's QIP2025 talk on reducing the space cost of quantum factoring:
www.youtube.com/watch?v=R3yM...
And her co-author Schrottenloher's talk at the Simons Summer Cluster on Quantum Computing:
www.youtube.com/watch?v=0pnX...
www.youtube.com/watch?v=R3yM...
And her co-author Schrottenloher's talk at the Simons Summer Cluster on Quantum Computing:
www.youtube.com/watch?v=0pnX...
Clémence Chevignard: "Reducing the Number of Qubits in Quantum Factoring" (QIP 2025)
YouTube video by QIP2025
www.youtube.com
July 30, 2025 at 10:31 AM
Chevignard's QIP2025 talk on reducing the space cost of quantum factoring:
www.youtube.com/watch?v=R3yM...
And her co-author Schrottenloher's talk at the Simons Summer Cluster on Quantum Computing:
www.youtube.com/watch?v=0pnX...
www.youtube.com/watch?v=R3yM...
And her co-author Schrottenloher's talk at the Simons Summer Cluster on Quantum Computing:
www.youtube.com/watch?v=0pnX...
My QIP2025 talk on magic state cultivation:
www.youtube.com/watch?v=bbR0...
A later longer version of the talk, given at the Simons quantum colloquium:
www.youtube.com/watch?v=gxzt...
www.youtube.com/watch?v=bbR0...
A later longer version of the talk, given at the Simons quantum colloquium:
www.youtube.com/watch?v=gxzt...
Craig Gidney: "Magic state cultivation: growing T states as cheap as CNOT gates" (QIP 2025)
YouTube video by QIP2025
www.youtube.com
July 30, 2025 at 9:46 AM
My QIP2025 talk on magic state cultivation:
www.youtube.com/watch?v=bbR0...
A later longer version of the talk, given at the Simons quantum colloquium:
www.youtube.com/watch?v=gxzt...
www.youtube.com/watch?v=bbR0...
A later longer version of the talk, given at the Simons quantum colloquium:
www.youtube.com/watch?v=gxzt...
People complain that the "find the cycle in a linked list" interview question is contrived.
I thought that too… til I was writing the loop analysis in stim's circuit-to-dem conversion, realized it was analogous, and the standard answer was way better than what I'd been writing.
I thought that too… til I was writing the loop analysis in stim's circuit-to-dem conversion, realized it was analogous, and the standard answer was way better than what I'd been writing.
July 15, 2025 at 3:45 PM
People complain that the "find the cycle in a linked list" interview question is contrived.
I thought that too… til I was writing the loop analysis in stim's circuit-to-dem conversion, realized it was analogous, and the standard answer was way better than what I'd been writing.
I thought that too… til I was writing the loop analysis in stim's circuit-to-dem conversion, realized it was analogous, and the standard answer was way better than what I'd been writing.
A circuit compilation exercise. Show that MX+CX+T is a universal gateset by constructing an H gate.
MX: X basis measurement
CX: controlled NOT
T: 45 degree rotation around Z axis
(Classical feedback and ancillas is allowed.)
MX: X basis measurement
CX: controlled NOT
T: 45 degree rotation around Z axis
(Classical feedback and ancillas is allowed.)
July 15, 2025 at 3:40 PM
A circuit compilation exercise. Show that MX+CX+T is a universal gateset by constructing an H gate.
MX: X basis measurement
CX: controlled NOT
T: 45 degree rotation around Z axis
(Classical feedback and ancillas is allowed.)
MX: X basis measurement
CX: controlled NOT
T: 45 degree rotation around Z axis
(Classical feedback and ancillas is allowed.)
A reminder that if your paper has open data containing stim circuits, I keep a list of those and you can be on the list. I know my 2025 list so far is incomplete: github.com/quantumlib/S...
Update circuit_data_references.md by Strilanc · Pull Request #975 · quantumlib/Stim
A fast stabilizer circuit library. Contribute to quantumlib/Stim development by creating an account on GitHub.
github.com
July 14, 2025 at 6:54 PM
A reminder that if your paper has open data containing stim circuits, I keep a list of those and you can be on the list. I know my 2025 list so far is incomplete: github.com/quantumlib/S...
Based on Zenodo stats, people like the cultivation and factoring papers way more than my others. In the same league as downloads of the below surface code threshold experiment data!
Could be recency bias (average rate should drop over time) but the totals are already higher.
Could be recency bias (average rate should drop over time) but the totals are already higher.
July 11, 2025 at 8:37 PM
Based on Zenodo stats, people like the cultivation and factoring papers way more than my others. In the same league as downloads of the below surface code threshold experiment data!
Could be recency bias (average rate should drop over time) but the totals are already higher.
Could be recency bias (average rate should drop over time) but the totals are already higher.
I found a nice way to draw multi-qubit-parity controls.
Lattice surgery likes accessing these kinds of operators, so it'd be counter-productive to decompose them using CX gates while hoping a later step would put them back together. Having notation that avoids that is handy.
Lattice surgery likes accessing these kinds of operators, so it'd be counter-productive to decompose them using CX gates while hoping a later step would put them back together. Having notation that avoids that is handy.
July 9, 2025 at 3:22 PM
I found a nice way to draw multi-qubit-parity controls.
Lattice surgery likes accessing these kinds of operators, so it'd be counter-productive to decompose them using CX gates while hoping a later step would put them back together. Having notation that avoids that is handy.
Lattice surgery likes accessing these kinds of operators, so it'd be counter-productive to decompose them using CX gates while hoping a later step would put them back together. Having notation that avoids that is handy.