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Comment by spenvo | Hacker News Reader

Who here missed https://news.harvard.edu/gazette/story/2023/12/researchers-c... Put simply: the breakthrough here (not related to the IBM news in parent link) massively cuts the physical qubits needed to do error-correcting (in the pursuit of logical qubits). What they demonstrated in this regard was vastly more efficient (more than the 1000:1 industry benchmark, though it's not apples to apples) than the current state of the art (as well as what IBM is hoping to achieve in its next generation chip). It seems to have pulled game-changing quantum computing years closer to reality. The paper: https://www.nature.com/articles/s41586-023-06927-3

Sorry if it's seen as off topic, my standalone submission on it didn't generate discussion https://news.ycombinator.com/item?id=38705445

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Don't get me wrong, their (QuEra's) demonstration is incredibly impressive, but it seems you've been misled by inconsistent nomenclature around the phrase "logical qubit". They've demonstrated a 5/1 encoding scheme, yes, but that scheme is not anywhere close to being sufficiently redundant to allow for deep quantum circuits. When people talk about needing 1000 physical qubits, they mean to make a logical qubit with sufficiently low error rate to run interesting algorithms. In the QuEra device, when they say they "made 48 logical qubits out of 240 physical qubits", they simply meant that they used an encoding, and made no claim about the error rate on those qubits being low enough. There is no hope (that I know of) for a 5-1 encoding scheme to make error rates low enough. The QuEra device would just as well need many more physical qubits per logical qubit.

I want to point out that the experiment was at Harvard in the Lukin group. There is a proposal for constant-rate encodings using large quantum low-density parity check codes via atom rearrangement which could in principle achieve such high encoding rate. That said, it's certainly not mainstream yet. https://arxiv.org/abs/2308.08648

Yes, good point (apologies to the Lukin group). That's an interesting proposal, but it seems from a cursory read that you would need still need very many physical qubits to approach that asymptotic rate, and also you would be forced to take a very large slow down due to serializing all of your logical operations through a smaller set of conventionally encoded logical qubits. That said, I'm not current on SOA LDPC QEC proposals, so I'll moderate my claim a bit to "the first actually useful logical qubits will almost certainly have an encoding rate lower than 1/5".

"Peanut Gallery" here... These types of conversations are the reason I'm still addicted to HN.

Thank you both. And /hattip

By your estimation, what does this practically mean in terms of: we may be able to do X Y and Z in ______ years now because of this development and other recent innovations in quantum computing.

I’m more curious about this from a consumer angle - anything with at least a downstream consumer impact that is exciting? And even if no consumer impact for the foreseeable future, is there anything “cool” or more tangible than massive encryption breaking?

The encryption angle is somewhat uninteresting. Old recorded messages will be broken eventually, quantum computing or not. Thwarting the quantum threat for new keys just requires a constant factor of additional work (key size, encryption cost). Thwarting it without much overhead is an ongoing research problem.

The ______ years question is everyone's best guess. X, Y, and Z though are quantum simulations. Drug discovery, superconductors, "better" alloys (use-case dependent), .... It's shockingly expensive to simulate a properly quantum system, and that cost increases exponentially in its size. 1 atom is manageable usually, ignoring relativity. 2 is an interesting research problem. 3 are tractable if they're homogenous and you're patient. Everything beyond that requires huge simplifications or clever insights (or both). Quantum computing linearizes that simulation cost, for any problem small enough to fit inside the given qubit bounds.

One way to estimate the effect might be to look at current chemistry-related AI/ML news. It's a noiser version of the same results. If that winds up being useful then quantum computers will be even more useful (but perhaps not worth it). If that doesn't advance materials research enough then it's less obvious that a quantum computer will be helpful. Maybe, maybe not.

> Old recorded messages will be broken eventually, quantum computing or not.

That may well take multiple lifetimes, greatly reducing its impact. Mere increases in computing power will not break 256 bit Elliptic Curve cryptography for centuries. That will require algorithmic breakthroughs or large scale quantum computers as well.

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