May 16, 2018 · 1h 33m · y-combinator
John Preskill on Quantum Computing · Y Combinator
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In this Y Combinator interview, theoretical physicist John Preskill provides a comprehensive overview of quantum computing, covering its historical origins, core principles like entanglement and error correction, competing hardware architectures, algorithm speedups, cybersecurity risks, commercial startup opportunities, and connections to fundamental quantum gravity.
How this conversation actually went
Every chapter scored 0–10 on four independent dynamics. Hover any point for the reasoning behind the score. How this is scored →
The partners as informed peer 2.6 Guest teaching 5.5 Guest disagreement 0.9 The partners pushing back 0.3
speaking balance: gold is the partners, purple is the guest (3 minute bins)
Sharpest disagreement ▶ 32:16 Preskill refuses horse race framing
When pressed to name the single winning hardware engineering strategy, Preskill flatly refuses the premise, noting that picking a winner today is premature.
Hardest push from the partners ▶ 34:52 Host pushes cloud abstraction modelCraig challenges the hardware-centric focus by re-anchoring the conversation around an AWS-style server-farm cloud abstraction layer.
Biggest teaching moment ▶ 1:02:22 Preskill dismantles mid-computation measurement fallacyPreskill directly corrects Craig's mistaken description of algorithmic gate steps, explaining that measuring intermediate states instantly collapses quantum superposition.
The partners hold their own ▶ 1:14:08 Craig matches empirical heuristics to machine learningCraig demonstrates sharp technological domain awareness by tying Preskill's thesis on heuristic algorithm discovery directly to modern machine learning empirical benchmarks.
the scores for every segment, with the reasoning behind each
| Chapter | Topic | The partners as informed peer | Guest teaching | Guest disagreement | The partners pushing back | Why |
|---|---|---|---|---|---|---|
| Quantum Entanglement and the Quantum Book Analogy | 1 | 6 | 1 | 0 | Craig asks for an explanation of quantum error correction. Preskill pauses to establish the foundational concept of quantum entanglement using the extended metaphor of reading a correlated quantum book. | |
| Mechanics of Quantum Error Correction and Decoherence Protection | 3 | 7 | 2 | 1 | Craig confuses error correction and gate measurement with Grover's algorithm. Preskill steps in with a thorough explanation of quantum interference and double-slit probability amplitudes. | |
| Practical Applications: Chemistry, Materials Science, and Carbon Capture | 3 | 5 | 1 | 0 | Craig relays audience questions regarding practical quantum advantages. Preskill outlines near-term commercial domains like catalysis, carbon capture, and quantum chemistry simulations. | |
| Exponential Speed-ups and Shor's Factoring Algorithm | 3 | 6 | 1 | 0 | Preskill contrasts polynomial Grover search with exponential speedups demonstrated by Shor's factoring algorithm, detailing why quantum chemistry is intractable classically. | |
| The Quantum Hardware Landscape and Near-Term NISQ Devices | 2 | 5 | 0 | 0 | Preskill reviews 50-to-100 qubit NISQ processors, highlighting the classical simulation boundary while clarifying why noisy gates limit immediate practical utility. | |
| Hybrid Algorithms, Gate Error Rates, and Hardware Implementations | 3 | 6 | 1 | 1 | Craig asks how error backup works in practice. Preskill explains hybrid classical-quantum feedback algorithms and details trapped-ion versus superconducting hardware physics. | |
| Topological Quantum Computing and Coherence Time Progress | 2 | 6 | 1 | 0 | Preskill discusses Microsoft's topological quantum computing ambitions and historical exponential gains in superconducting circuit coherence times. | |
| Engineering Strategies, Hardware Leaders, and Cloud Stack Ecosystems | 4 | 4 | 2 | 1 | Preskill declines to declare a winning hardware modality. Craig introduces an apt parallel to the AWS cloud stack, which Preskill agrees will dominate near-term access models. | |
| Room-Temperature Quantum Technology and Public Key Cryptography Vulnerabilities | 3 | 6 | 2 | 1 | Craig asks about quantum pocket devices at sub-Kelvin temperatures. Preskill jokes about the thermal impossibility before breaking down RSA and elliptic curve crypto vulnerabilities. | |
| Quantum Key Distribution and Quantum Repeaters | 2 | 6 | 1 | 0 | Preskill explains quantum key distribution (QKD) and clarifies why optical fiber transmission requires specialized quantum repeaters that do not measure transit photons. | |
| Quantum Sensing and Business Opportunities for Startups | 2 | 5 | 0 | 0 | Responding to audience startup questions, Preskill explains how qubit environmental sensitivity can be inverted into high-precision biomedical quantum sensing. | |
| Quantum Gravity and Emergent Space-Time Geometry | 2 | 7 | 0 | 0 | Preskill details his theoretical work on the entanglement frontier, suggesting space-time geometry itself emerges from underlying quantum error-correcting network correlations. | |
| Dispelling Misconceptions: Entanglement, Teleportation, and Interference | 3 | 8 | 3 | 1 | Preskill debunks common media fallacies around faster-than-light teleportation and parallel universe computation, gently correcting Craig's premise on mid-circuit measurement. | |
| Personal Anecdotes of Working with Richard Feynman at Caltech | 2 | 4 | 0 | 0 | Preskill shares fond personal memories of Richard Feynman at Caltech, noting Feynman's first-principles stubbornness and hallway bongo drumming. | |
| Teaching Quantum Intuition to Youth and Empirical Algorithm Discovery | 4 | 5 | 0 | 0 | Preskill argues quantum mechanics intuition can be built through interactive gaming. Craig links empirical algorithmic discovery to current deep learning trial-and-error workflows. | |
| STEM Education, Critical Thinking, and Scientists in Public Policy | 3 | 4 | 1 | 0 | Preskill advocates for STEM education focused on evidence-based critical thinking and reviews physicists in public governance, noting past Energy Secretaries. | |
| Quantum Valley and Assembling Cross-Disciplinary Technical Teams | 2 | 4 | 0 | 0 | Preskill reflects on the emergence of quantum hardware startups in Silicon Valley and stresses the necessity of cross-disciplinary communication between software and microwave engineers. | |
| Pedagogy, Recommended Learning Resources, and Teaching Insights | 3 | 5 | 0 | 0 | Preskill recommends Leonard Susskind's educational work and recounts how teaching forced him to master quantum complexity theory and quantum chromodynamics from scratch. |
Statements from this episode (35)
Preskill: Quantum error correction is now achievable in the laboratory
“But at first that was just kind of a theorist fantasy. It was a little too far ahead of The technology, but, you know, 20 years later, the technology is catching up, so now this idea of quantum error correction has become something you can do in the lab.”
Preskill: Entanglement is the primary distinction between quantum and classical physics
“It's really the characteristic way, maybe the most important way that we know, in which quantum is different from ordinary stuff. Ok. From classical.”
Preskill: Quantum error correction hides data from noise via entanglement
“So we have to encode the information so the environment, so to speak, can't find out anything about what the information is. And that's the idea of quantum error correction. If we encoded an entanglement, The environment is looking at the parts one at a time, …”
Preskill: Grover's algorithm only doubles tractable cities in traveling salesman problems
“And in practice, it's not that big a deal. What it means is that, you know, if you have the same processing speed, you can handle about you know, twice as many cities before the problem becomes too hard to solve as you could if you were using a classical proce…”
Preskill: Most Important Quantum Applications Have Not Yet Been Conceived
“Probably the most important ones are things we haven't thought of when it comes to applications of quantum computing.”
Preskill: Quantum computers will impact chemistry, carbon capture, and materials
“The Ones which will affect everyday life, I think, are better methods for understanding and inventing new materials, new chemical compounds. Things like that can be really important. You know, if you find a better way of capturing carbon by designing a better …”
Preskill: Efficient classical algorithms for quantum chemistry are unlikely
“Maybe some brilliant graduate student is going to drop a paper on the archive tomorrow which will say, here I solve quantum chemistry and I can do it on a digital computer. But we don't think that's very likely because we've been working pretty hard on these p…”
Preskill: Current quantum computers are too small for proposed algorithms
“We have algorithms that have been proposed, but, which we can't really run currently, because our quantum computers aren't big enough, on the scale that's needed to solve problems really, people really care about.”
Preskill: Quantum hardware is far from converging on a single winning approach
“There are a number of different approaches to Building hardware, and nobody really knows which is going to be the best. We haven't, I think we're far from collapsing to one approach, which everybody agrees has the best long-term prospects for scalability.”
Preskill: 50-to-100 qubit quantum computers will arrive in a couple years
“We think in a couple of years we'll have devices with about 50 qubits to a hundred, and we'll be able to control them pretty well, and that's an interesting range, because even though it's only 50 to a hundred qubits, doesn't sound like that big a deal, but th…”
Preskill: Near-term quantum computing has best hopes in hybrid classical-quantum methods
“Well, I think in the near term, we're going to be trying out, and probably we have the best hopes for kind of hybrid classical quantum methods with some kind of classical feedback.”
Preskill: Current quantum tech achieves gate error rates around a few parts per thousand
“More or less with the technology we have now, you can have a gate error rate of a few parts in a thousand, you know.”
Preskill: Validated topological qubit may appear as soon as 2019
“And it's very ambitious, because at this point, it's not even clear they have a single qubit, but if that approach is successful, and we're, it's making progress, so I think we will see a validated Qubit of this type soon, maybe next year.”
Preskill: Superconducting qubit coherence times grew 10x every three years
“And for the superconducting circuits, those coherence times have increased about a factor of 10 every three years, going back 15 years or so.”
Preskill: Superconducting circuits and trapped ions will lead for 5-10 years
“The most advanced are superconducting circuits and trapped ions, which is why I mentioned those first, and I think that will remain true, you know, over the next five to 10 years.”
Preskill: Superconducting circuits execute gates faster than trapped-ion qubits
“They have the advantage of being faster than the time to take the cycle time, the time to do a gate is faster than with the trapped ions.”
Preskill: Silicon electron spin qubits could surpass other modalities long-term
“In the long term those electron spins could catapult ahead of these other things. That's something that you can naturally do in silicon, and you know, it's potentially easy to integrate, integrate with silicon technology.”
Preskill: Quantum computers can break both RSA and elliptic curve cryptography
“RSA, which is one of the ones that's widely used, as typically practiced today, the, to break it, you'd have to do something like factor a number, which is Over 2000 bits long to 20 48. And that's, you know, that's too hard to do now. But that's what quantum c…”
Preskill: Quantum computers breaking public key crypto unlikely in 10 years, likely in 50
“I don't think, though I could be wrong, that we're likely to have quantum computers that can break those public key cryptosystems in 10 years. But in 50 years? Seems not unlikely.”
Preskill: Quantum information can travel tens of kilometers in fiber with low error
“But nowadays, you know, you can send quantum information through an optical fiber over tens of kilometers with you know, a low enough error rate, so it's useful for communication.”
Preskill: Single photons cannot reach global distances in fiber without repeaters
“Because at least with, you know, our current photonics technology, there's no way I can send a single photon from here to China without there being a very high probability that it gets lost in the fiber somewhere.”
Preskill: Quantum repeaters require much smaller processors than computing hard problems
“And so it will require some quantum processing to get that quantum error correction in the quantum repeater to work. But it's a much more modest scale quantum processor than we would need to solve hard problems.”
Preskill: Nuclear-spin quantum sensors can exceed existing spatial resolution limits
“And again, using this technology of Nuclear spins, which I mentioned you can do it at room temperature, potentially. You can make a pretty good sensor, and it can potentially achieve higher sensitivity and spatial resolution. You look, look at things on shorte…”
Preskill: Quantum entanglement is what holds space-time geometry together
“So it's really the entanglement which holds space together, which keeps it from falling apart into little pieces.”
Preskill: Quantum computers will enable tabletop toy spacetime in decades
“And in particular, what we're going to be doing with quantum computers and, you know, the other quantum technologies that are Becoming increasingly sophisticated in the next couple of decades is we'll be able to control very well, highly entangled, complex qua…”
Preskill: Quantum entanglement does not enable faster-than-light communication
“What sometimes Gets said, or the impression people get, is that that means that when I do something to my qubit, it instantaneously affects your qubit, even if we're on different sides of the galaxy. But that's not what entanglement does. It just means they're…”
Preskill: Quantum speedup requires engineering interference to cancel incorrect answers
“The art of a quantum algorithm is to make sure that the wrong answers interfere and cancel one another out, so the right answer is enhanced. And that's not automatic. It requires that the quantum algorithm be designed in just the right way.”
Preskill: Feynman Insisted on First Principles Over Expert Literature
“Feynman he always wants to think things through for himself. From first principles rather than rely on the guidance from experts who have thought about these things before.”
Preskill: Richard Feynman's refusal to read 1980s literature hurt his research
“The big difference between Feynman and me in the mid-nineteen-eighties is, I was reading literature, and he wasn't. And probably if he had been, it would have, he would have been well-served.”
Preskill: 10-year-old kids will play quantum games in a few decades
“What I wouldn't be surprised by is that if you go out a few decades, kids who are 10 years old are gonna be playing quantum games.”
Preskill: Many classical computer algorithms were discovered empirically through experimentation
“A lot of classical algorithms that, that people use on today's computers were discovered, or that they were powerful, was discovered by experimenting, you know, by trying it.”
Preskill: STEM education should prioritize critical reasoning over workforce training
“That STEM education, we shouldn't think of it as, we're going to need this technical workforce, and so we better train them. I think the key thing is, we want the general population to be able to reason effectively. You know, and to recognize when an argument …”
Preskill: Silicon Valley is likely to become Quantum Valley
“That's the place with the Silicon Valley is likely to be Quantum Valley the way things are right now.”
Preskill: Quantum startups should prioritize cross-disciplinary hires
“Being able to communicate across those boundaries is very valuable, and you can see it in quantum computing now, that if the man or woman who's involved in the software has that background, but there's not a big communication barrier talking to the people who …”