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Comment by marricks | Hacker News Reader

> He said: “This is a very nice demonstration of quantum advantage. While a great achievement academically, the algorithm used does not really have real world practical applications, though.

> “We really must get to utility quantum computing – an era where quantum computers with many thousand qubits actually begin to deliver value to society in a way that classical computers never will be able to.”

This seems to be the most level headed take from the paper and an explanation why (if claims are to be believed) we haven't seen anything useful come from quantum computing yet that couldn't be done by classical computers.

Maybe it still is all hype and the reason we haven't seen a useful calculation is because the only problems it can solve better than others happen to be especially contrived. I guess time will tell.

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I don't think it's that the concepts are contrived (although the test cases certainly are), it's just that our quantum computers are really early in their development and can't do the more complicated things yet. It's like if we had calculators that took an hour to do each arithmetic operation- the fact that people wouldn't use it doesn't mean arithmetic is contrived, just that it isn't as powerful as better alternatives.

As the quote mentions, we'll need "many thousand qubits" to reach the point where society itself is getting new benefits out of things. These contrived cases help figure out how to scale the whole system.

It's more like if we had calculators that could not do any arithmetic operation at all...

Someone someday might figure out how to make the calculator actually calculate, but not yet.

There's a fundamental question as to whether or not it's exponentially difficult to add additional qubits. If each marginal qubit is 5% more difficult to add as the previous one the task would be essentially impossible - and even if it wasn't impossible it would turn out to basically be cheating (in that you would be doing exponential work via either the quantum computing route or the conventional route).

Note: that there is good physical reasons why the cost of QC may grow exponentially with qbits. Refrigeration is exponentially inefficient as T=>0 and the gap of a quantum system which sets the temperature you must cool to shrinks as you couple new degrees of freedom. This dynamic has been the basic reason for the sub exponential progress in the area (despite exponential expenditure)

Not for photonic quantum computing. Only detectors require cooling, and it is possible to build adequately sized quantum computers with constant number of detectors using loop based architectures.

Even more realistic architectures are very very cost effective on the number of components https://quantumfrontiers.com/2023/06/21/what-is-the-logical-...

Yeah, it always struck me as odd that the main quantum computing research labs went for the "VLSI" approach straight away rather than building room-sized computers with qbits that are known to be more stable and don't require such aggressive refrigeration.

Between the costs of refrigeration/fabrication and the increased speed of decoherence, it's not hard to imagine that the approach of using superconducting qbits may be a dead end, despite quantum ECC.

There are very valid reasons for this. Historically, this is what happened.

* People did very basic qubit experiments with NMR in the late 90s. Very noisy experiments.

* Around 2000 they realized photonic quantum computers would be "easiest" because light experiences very little noise. The problem was that its insanely difficult to do non-linear interactions between photons (which is necessary to do any sort of non-trivial classical or quantum computing with photons). In 2001, somebody came up with a clever way of doing non-linear interactions by using fast detectors.

* Huge efforts started to try and build photonic quantum computers. Unfortunately, around 2004-05, people started to do estimates and it turned out that the number of sources and detectors needed with the clever way was humongous. Far more than we could hope to achieve, and there didn't seem to be any way of reducing it. People abandoned photonic quantum computing and started doing ion traps and superconducting.

* Interestingly around the same time in 2005, there emerged an alternate method of building photonic quantum computers, based on "cluster states". However, the method also had the same humongous resource problem, but it had an advantage: the framework could be modified and played with to improve it. Over the next two decades, very slowly people figured out improvement after improvement to this architecture to bring down the resource costs.

* At this point, this cluster-state photonic architecture has improved quite a bit and is starting to become very competitive with ion traps and superconducting qubits. PsiQuantum (whose article I shared above) is the leader in this right now. And they might win the race.

Yes, but all existing photonic platforms use post-selection which is even more clearly exponentially lossy. Although this could be solved with a deterministic single photo source if one can be found. Photonic quantum computing is an especially funny post-transistor paradigm because classical photonic computing is also quite attractive.

If you read the linked article, it explains how to avoid exponential loss on post-selection.

> Imagine you can toss coins, and you need to generate 20 coins showing Heads. If you repeatedly toss all 20 coins simultaneously until they all come up heads you’d typically have to do so millions of times before you succeed. This is even more true if each coin also has a 20% chance of rolling off the table (akin to photon loss). But if you can toss 20 coins, set aside (switch out!) the ones that came up heads and re-toss the others, then after only a small number of steps you will have 20 coins all showing heads. This large gap is fundamentally why the first whammy is not relevant: To generate a large photonic entangled state we begin by probabilistically attempting to generate a bunch of small ones. We then select out the success (multiplexing) and combine successes to (again, probabilistically) generate a slightly larger entangled state. We repeat a few steps of this. This possibility has been appreciated for more than twenty years, but hasn’t been done at scale yet because nobody has had a good enough optical switch until now.

Haha a self promotional blog post without any experimental data from a lab self-interested in the technology is somehow not persuasive to me.

There is no way to add entangled particles to an entangled photon state without new light matter interaction which may (and almost certainly) bring decoherence.

Ie to claim a good enough switch is possible is a claim demanding a ton of evidence, and there is none right now.

Ofc I would be thrilled to see it work! But explaining a scheme in back of the envelope fashion and measuring it are two vastly diff things.

The terms "realistic" and "cost effective" in the field of quantum mechanics do not have scalar definitions. The meaning changes depending on the problem you are working on and the deadline for funding opportunities.

This is not quite true. You only need to keep the qubits at a fixed temperature as you scale the system, so the resources required to add additional qubits grow only polynomially with the system size. Once you have many qubits with a sufficiently low (but constant) error rate, you can do quantum error correction which also only has polynomial overhead.

No you are missing the fact that if the qubits are coupled the systems fundamental gap has shrunk demanding a lower temperature for the same error rate.

You should think more about why your rosy scheme hasn't worked yet if you can't explain that empirically maybe you don't quite understand.

The qubits aren't all coupled all the time. The whole point of a 2-qubit gate is the coupling is controllable. Also -- why is the gap of the bare system important? The whole point of QEC is the creation of a decoherence free subspace. Your model is wrong and you don't understand quantum error correction.

The qubits aren't meant to be coupled, but in practice everything is coupled at some level. I find it plausible that having a system with tons of degrees of freedom on a limited energy scale might make it difficult to isolate all the degrees of freedom from each other.

Yeah but these are the most basic of basic problems in QC and people would be discarding approaches that didn’t have potential solutions to this issue. In superconducting QCs, neighboring qubits are not necessarily even resonant, and are in their own little metal boxes. With neutral atoms, you have extremely long lived internal states that barely couple to the environment, and the atoms are macroscopic distances from each other.

> ... quantum computers are really early in their development and can't do the more complicated things yet. It's like if we had calculators that took an hour to do each arithmetic operation ...

Reminds me of the story George Stibitz tells after developing one of the first devices that could be called a computer. Management at Bell Labs was not impressed that he had built a "20000 dollar calculator".

https://youtu.be/qundvme1Tik?t=259

Well a “20000 dollar calculator” is something QC can only dream of

The field of quantum computing HAS to have incremental results, or else nobody will continue to fund it. Unfortunately, it doesn't seem to be presenting great opportunities for incremental results. This doesn't invalidate the field, it just means that it's really hard to tell if you're going to see a pay-off. These things aren't really factored into the risks that investors think they're taking, so it's giving me a weird feeling in the pit of my stomach. I desperately want to see quantum computing work -- I've worked on it for 15 years now. From my vantage point, I see progress, but that's progress in making better qubits, better gates, better architectures, not solving problems. There's an error correction threshold, remember, and you start seeing real results when/if you cross it.

Anyhow, with all these huge beautiful dilution fridges with their fancy gold plating and all that, the expense of QC is a rounding error relative to AI. It's still high-risk/high-reward and has seen a level of investment that's commensurate with that.

Also, I don't know who all thinks that these physicists like myself are making beaucoup bucks working on QC. I make far less than a software engineer at grubhub or something, and have taken the opportunity cost of getting a PhD and all that.

Quantum computing feels like nucleair fusion. Been hearing about how its going to change the world since I was kid.

Well, in fairness, I could say the same about solar, improved batteries, etc.

> solar, improved batteries

Neither of those sounded as sexy (i.e hyped) to me a nuclear fusion and quantum computers.

> solar, improved batteries

>> Neither of those sounded as sexy (i.e hyped) to me a nuclear fusion and quantum computers.

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See, solar can be sexy!

Eh? A random person making up a fictional press release doesn't count as hype, you know that right?

Solar has become incredibly inexpensive in the last decade, and batteries likewise are many times better than they were 20 years ago. Is there further to go, yes, but progress has been steady and real applications are all around you for both. Our phones are marvels of battery technology that was impossible 15 years ago.

Fusion and quantum computing on the other hand. Neither has yet solved one problem or made net progress. They have great promise but the practicalities may ultimately outweigh the benefits. Meanwhile, solar-charged batteries powering high performance electric cars are more common every day.

You think there has been no net progress in QC or fusion? There has been a lot of progress, but not to the point where we can make practical use of them yet.

Some problems are just harder than others. Maybe some of them are not soluble, or some are not economic. We won't know if we don't try, and they hold out great promise.

Quantum computing has been just as sexy a topic as stable fusion.

Those were/are goalposts. We've never actually /seen/ the goal though, and the posts are a collective hallucination. It IS a pretty nice hallucination though.

Since we as a species don't quite understand quantum interactions, it could be quite some time before we can build ordered and predictable systems out of those interactions.

It could take days/weeks/months/years/decades/millennium... who knows? The fun is in watching and playing.

It’s a milestone on a roadmap. Science like this doesn’t happen in eureka moments - it’s a methodical slog through our ignorance where we latch in wins bit by bit over decades. Quantum computing, AI, fusion, curing cancer, ending aging, they’re pinnacle achievements at the extended play of Civilization.

A potential hidden bonus: if quantum computing theories get enough attention, maybe people will start to consider using the quantum computing abilities of the human mind with conscious intentionality (since by the time we get computers to be able to do it, it may be too late)!

What are these capabilities of which you speak? Or have i missed the sarcasm

In many other worlds gp got tremendous karma for their post. That's how quantum mental abilities work.

Do you mean; Use a quantum random number generator and emit random words. Changes are that in some world the comment is insightful?

Well, generate reality for one. It's not a trivial detail, but it tends to be dismissed or taken for granted (or downvoted lol).

Someone may venture into this territory some day, and perhaps that someone will find some travelling companions to make the journey more exciting and productive....time will tell!

The trick is you can only flip as yet unobserved bits. If we know the asteroid is coming we can't delete it.

Some bits can even be flipped in broad daylight while everyone's watching. One of the most popular ways to flip bits is by telling stories in which the bits have been flipped.

The power of stories is that you are alone in the telling of them. The survival, your own undressing, or redressing, the terrible fury of living life, constrained to the teacup of bedtime for children. The power that storytelling takes is a kind of leadership that is grudgingly respected, like a real bastard's funeral.

It doesn't have much to do with physics.

> The power of stories is that you are alone in the telling of them.

I don't know about you, but I've noticed a pattern where the stories people tell on social media and even in person often have an uncanny resemblance to the stories that are told the day before in mainstream media.

Heck, if everyone plays their cards right, you can often even get a nice (profitable) war going!

> It doesn't have much to do with physics.

Mostly agree (the jury is still out on the hard problem of consciousness), it's metaphysics... and we all "know" what that "means": "woo woo"...or so they say.

And, that realm often leaks into the physical realm, much to our horror/delight/confusion.

Generate reality? I am not crystal clear on what you're talking about. Quantum computers are still just computers, not magical replicators from Star Trek.

I'm talking about the mind's ability to generate what "is", and "is not". It is a curso no doubt, but it is also sometimes a blessing. But usually it runs unnoticed (or even: denied) in the background, duffing things up.

And when it gets noticed, sometimes really weird things happen.

> And when it gets noticed, sometimes really weird things happen.

....such as?

If you have any proof you can probably get a nobel prize.

Everyone "already knows" these things, so no awards or even surprise awaits one.

If you can change bits that have already been observed, that would be very surprising to people.

Otherwise I'm not sure what you mean by "generate" but almost any kind of influence would have massive potential.

Surprising to some, pleasing to others, infuriating to others. But to those who have mastery over it: powerful.

I was thinking op meant something like realizing everyone has their own take on NULL and none of them are really wrong or right.

This is pretty close...although some can be correct, but correctness is not necessarily justified (such as when guessing what "is probably" true, which is why the J in JTB exists, and perhaps also because people like to take the T for free, as doing otherwise "is pedantic", which almost always has consensus agreement - reality domes don't maintain themselves!!).

But where does the quantum come in? You don't need it to have/manipulate consensus.

What are the "weird things"?

> But where does the quantum come in?

My thinking is here:

https://en.m.wikipedia.org/wiki/Quantum_superposition

> You don't need it to have/manipulate consensus.

Right, it's obliviousness to (or ideological, indoctrinated denial of) the phenomenon of extra variables/state that enables the manipulation of consensus. (I'm looking at you, scientists.)

Also, if these higher states did not exist, I'd think the problem might be either be impossible in the first place, or impossible to detect. (Noteworthy: having flawed methodologies can also make them impossible to detect.)

> What are the "weird things"?

Hallucination is the most obvious, but I would also say: just look around!

Or for a more serious analysis:

https://slatestarcodex.com/2014/07/30/meditations-on-moloch/

> (or downvoted lol)

Just use your quantum mind to change that downvote to upvote, problem solved.

I’m guessing he’s referring to the Roger Penrose stuff — https://en.m.wikipedia.org/wiki/The_Emperor%27s_New_Mind , etc.

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