May 25, 2018 · 1h 18m · y-combinator
Simon Benjamin on Architectures for Quantum Computing · Y Combinator
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In this Y Combinator interview, Oxford quantum theorist Simon Benjamin explores the physics, hardware architectures, scaling challenges, and commercial prospects of quantum computing. He details how breakthroughs in error correction, ion-trap hardware, and modular optical networking are transforming theoretical physics into a practical engineering reality.
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 3.1 Guest teaching 5.8 Guest disagreement 0.4 The partners pushing back 1.2
speaking balance: gold is the partners, purple is the guest (3 minute bins)
Sharpest disagreement ▶ 1:13:10 Critique of quantum supremacy hype
Simon dismisses the commercial hype surrounding 50-qubit supremacy as an empty 'underpants gnomes' profit plan that ignores the lack of useful algorithms at that scale.
Hardest push from the partners ▶ 21:01 Questioning network link error exposureThe host directly challenges Simon's pitch for networked quantum architectures by questioning if arbitrary remote connectivity exposes the system to higher error rates.
Biggest teaching moment ▶ 11:14 Ancilla qubit measurement mechanicsSimon educates the host on how error-detecting ancilla qubits decouple error syndrome extraction from reading the protected logical quantum state.
The partners hold their own ▶ 50:48 Countering code breaking with quantum cryptographyThe host displays domain knowledge by countering Simon's code-breaking discussion, pointing out that quantum communication principles simultaneously provide provably secure defense.
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 |
|---|---|---|---|---|---|---|
| Recent Momentum and Commercial Interest in Quantum Computing | 2 | 5 | 1 | 1 | The host asks open-ended questions about the sudden momentum and laboratory breakthroughs behind quantum computing. Simon explains the core fragility of qubits and Oxford's 99.9% two-qubit gate fidelity record. | |
| Quantum Error Correction and Ancilla Qubit Detection | 4 | 7 | 0 | 2 | Simon details quantum error correction, explaining how ancilla qubits check for flips without directly measuring and collapsing the logical qubit. The host asks perceptive questions about measurement verification ('who guards the guards'). | |
| Fault-Tolerance Thresholds and Closing the Theory-Experiment Gap | 3 | 6 | 0 | 1 | Simon describes the fault-tolerance threshold historically moving from 99.9999% to roughly 99%, closing the gap with lab achievements. The host contributes clarifying points about algorithmic errors and qubit count scaling. | |
| Topological Surface Codes and Hardware Neutrality | 3 | 6 | 1 | 2 | Simon explains topological 2D surface codes and why nearest-neighbor connectivity avoids costly swaps. The host asks whether long-range network links introduce more errors into the system. | |
| Oxford Ion Trap Systems and Schrödinger's Cat Analogy | 2 | 6 | 0 | 0 | The host asks about Oxford's specific hardware approach, prompting Simon to unpack trapped-ion systems via a detailed Schrödinger's cat thought experiment regarding macroscopic superposition. | |
| Trapping Ions in Vacuum and Measuring Decoherence Times | 3 | 6 | 0 | 1 | Simon explains how calcium ions stripped of electrons are trapped in ultra-high vacuum above gold microchips, demonstrating 50-second decoherence times at room temperature. The host seeks quick clarification on temperature conditions. | |
| Long-Running Computations and the Scaling Challenge | 3 | 5 | 1 | 1 | The host asks how long calculations run and whether qubits are swapped out. Simon clarifies that fast gate errors dominate over natural decoherence and frames scaling as the true remaining bottleneck. | |
| Quantum Supremacy and Classical Simulation Limits | 4 | 6 | 1 | 1 | Simon breaks down the 50-qubit threshold of quantum supremacy, noting the exponential RAM requirements needed for classical supercomputers to simulate full state vectors. The host actively follows along, comparing supremacy to AGI narratives. | |
| The Utility Gap: Cryptography versus Molecular Discovery | 4 | 6 | 0 | 2 | Simon contrasts millions of qubits needed for Shor's algorithm with near-term NISQ molecular simulation. The host raises an insightful counterpoint regarding quantum cryptography defending against quantum decryption. | |
| Modular Quantum Architectures and Entanglement Distillation | 3 | 7 | 0 | 1 | Simon outlines his modular architecture proposal: linking small 5-qubit ion traps via photonic interconnects and using entanglement distillation to purify noisy optical channels into high-fidelity Bell pairs. | |
| Optical Link Speed, Server Farm Scale, and Commercial Development | 3 | 5 | 0 | 1 | The host presses on whether modular networking suffers latency compared to monolithic 50-qubit chips. Simon explains photon loss, optical cavities, and how quantum server farms could scale without re-engineering chip physics. | |
| Quantum Hype Cycles, AI Winters, and Career Entry Points | 3 | 5 | 1 | 1 | Simon warns against premature commercial hype and potential quantum winters, drawing parallels to AI history. The host agrees based on his machine learning contacts before asking how non-physicists can enter the field. |
Statements from this episode (26)
Benjamin: Commercial interest in academic quantum research saw a sea change
“Just in the last three years, we've had tons more interest from companies who come to us as academics and say, look, can we jointly work on something with you? I mean, that's really been a sea change.”
Benjamin: Ancilla qubits detect errors without collapsing main quantum data
“Basically this trick of not looking for errors where they are, but looking to one side, because you've basically done a separate little calculation that now in the, what we call ancilla qubits, it tells you just one thing, the one thing you do legitimately nee…”
Benjamin: Quantum computer design must treat all hardware operations as untrustworthy
“We must assume that everything is untrustworthy. Some things are worse than others. So measurement of a single qubit might be a bit more reliable than say the two qubit gate, but they all have a number on them that says how dodgy they are.”
Benjamin: 1990s quantum fault-tolerance threshold required 99.9999% fidelity
“And now the threshold when these results were first discovered in the nineties was about 10 parts per million. So I have a many nine, 99.9999 was the kind of level of precision that you would have to have in order to control your quantum computer and take the …”
Benjamin: Quantum error-correction threshold is now around 99% fidelity
“So now the threshold is about 99%. If you've got 90, if things work correctly in your quantum computer, 99% of the time, that's the turning point.”
Benjamin: Oxford quantum researchers achieve 99.9% gate fidelity
“But 99.9, which is what the guys here in Oxford can do, is 10 times better than the threshold.”
Benjamin: Kitaev surface code is the go-to quantum computer architecture
“Especially something called the Kataev surface code, which is, at the moment, the go-to solution for how we would build a quantum computer.”
Benjamin: Topological codes brought quantum fault tolerance down to 99% threshold
“So that was responsible primarily for moving from the multiple nines to the 99% threshold.”
Benjamin: Oxford Has ~200 Quantum Researchers in World-Leading Facility
“Oxford is actually one of the biggest research facilities in the world for quantum. We've got something like 200 people working on this, and so we're working on a bunch of different stuff.”
Benjamin: Oxford Holds World Record for Ion Trap Quantum Control
“But the one that I mentioned that we have the world record for the best control, is this thing called an ion trap, which sounds very technical.”
Benjamin: Trapped-ion qubits achieve 50-second to 10-minute coherence times
“So for the superconducting qubits that many researchers are excited, and we do do work on that here in Oxford as well the decay time, the amount of time that can go by before the wonderful zero one superposition just degrades, is the tiniest fraction of a seco…”
Benjamin: First useful quantum algorithms in the coming year will skip error correction
“Now, for the first quantum algorithms that we made, you know, the first serious ones that might do something useful, that perhaps we'll see in the coming year, ah, that's probably going to be the approach.”
Benjamin: Ion traps are the gold standard of qubits
“So they are actually the gold standard of qubits.”
Benjamin: Google, IBM, and Intel are primarily pursuing superconducting qubits
“In fact, if you look online, you will hear more about the superconducting qubits, which is the approach, the main approach that Google and IBM and even Intel are taking.”
Benjamin: Simulating 45 qubits classically requires 0.5 petabytes of RAM
“So, 45 qubits required 0.5. This wasn't my work. This was work elsewhere in the community. It required 0.5 half a petabyte of RAM, basically.”
Benjamin: Quantum applications needing fewer than 50 qubits can be simulated classically
“There's no point, if you think you've got a clever idea for what to use a quantum computer for, and that's going to change the world, if your idea involves much less than 50 qubits, you're wrong. Because, ah, you may have a very nice idea, but what we would do…”
Benjamin: Theorists have found no useful application for a 64-qubit quantum computer
“The theorists have not worked out anything that a 64 qubit quantum computer can do that's super useful.”
Benjamin: Quantum codebreaking requires millions of physical qubits due to error correction
“Breaking codes is in the category of things that needs at least thousands of qubits. But because it's a big, tough, long-running task, it also needs this whole error correction thing to be going on. And that boosts the size of it, because once you say, oh, wai…”
Benjamin: Building million-qubit quantum computers could take over a decade
“What we've worked out on paper is stuff that needs pretty big computers, and we don't know how long it would take to get there. You know, maybe, hopefully not decades, but it could take definitely more than a decade to get to the point where we have millions o…”
Benjamin: 50-qubit supremacy devices will launch within a year but lack practical utility
“On the other hand, what we believe is about to happen in the coming year is that people will start to bring out qubits, ah, sorry quantum devices that are at or just a little bit over the quantum supremacy threshold. This number, a lot of people are racing to …”
Benjamin: Quantum computing labs worldwide currently operate at 10 to 20 qubits
“Where we're at right now is that various labs around the world can give you 20 qubits. Ish. 10 to 20.”
Benjamin: Entanglement distillation allows low-quality links to achieve high internal fidelity
“As long as your boxes have a very good memory, they do have a very good memory, as long as they have very good quality internal operations, which they do have, then you can take a poor quality link and boost it by using it a few times into effectively a very g…”
Benjamin's team plans to scale using networked five-qubit modular units
“We actually are planning to build a machine, which will be made out of modules, which each one is a small quantum computer, and by the way, so small that on its own, it's not good for anything. Even as few as five qubits, right? And then we have modules, and w…”
Benjamin: Modular architecture is the only viable route to rapid quantum scaling
“And in fact, this network approach is, I think the only one that if you wanted to Manhattan project it, right, or moonshot it, you're one of these big projects that have happened in the past where you have a goal that's extremely challenging and you just decid…”
Benjamin: Oxford team aims to demonstrate linked two-module quantum computing within a year
“Sorting out thoroughly and demonstrating this idea of two modules, which fully link together to form a single unit in practice. So, you know, to the programmer, it's a single quantum computer. The engineer knows that it's been broken into two pieces with an op…”
Benjamin: Quantum hardware will soon reach behaviors impossible to classically simulate
“We are now getting on the verge of getting machines that will behave in ways that cannot be predicted, cannot be simulated. We'll be in the regime where we're genuinely discovering how the machine behaves by having one.”