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Comment by baq | Hacker News Reader

> IBM Quantum System Two is designed to work with IBM's future 433-qubit and 1,121 qubit processors.

what's the smallest useful (as in, 'non-toy', or maybe 'worth buying time on') quantum computer?

If you are a researcher from another field that just wants to contract out the computation of a numerical solution to some chemistry problem infeasible on a classical supercomputer, a million "physical" qubits is a fairly reasonable guestimate.

If you are a quantum computation person developing near term applications, you probably would already start getting excited with a 100 (sufficiently long-lived) qubits.

The "sufficiently long-lived" is the problematic part. Every lab has its own bespoke figure of merit (quantum volume, CLOPS, fidelities, etc). It is basically impossible to compare devices without being a researcher in the field for now. But at some point a novel drug or material will be developed thanks to a quantum computer and then we should really get excited about renting time on these devices.

>a million "physical" qubits is a fairly reasonable guestimate.

...

>If you are a quantum computation person developing near term applications, you probably would already start getting excited with a 100 (sufficiently long-lived) qubits.

...

>"logical qubit" or less formally "long-lived qubit" ... With 100 logical qubits (i.e. 100k physical qubits)

lol you're literally guilty of playing the same trick that people are bemoaning in another thread.

incidentaly, having taken a QC systems class from Fred Chong, i believe you guys are all working on vaporware.

Read the second comment you are quoting to the end. I think you misread something.

This your lede answer to what is the smallest useful non toy QC:

>If you are a quantum computation person developing near term applications, you probably would already start getting excited with a 100 (sufficiently long-lived) qubits.

Then you go on and on and on and have one sentence about what you can do with 100 full stop period qubits.

So what exactly did I misunderstand?

It's just funny to me how all of you guys - from the crypto QC grifter, all the way to PIs and postdocs like you play the same word game

I guess I should be more careful when using handwavy terms. "Long-lived" does not have a defined meaning, so in my second answer I spelled out what you can do with a bunch of "logical" qubits and with a bunch of "physical" qubits, providing clear definitions of these words. I am not sure why you are so angry about the use of a handwavy term though, given the whole point of this thread was to introduce the topic to lay people. Either way, the second comment is clear and it avoids the ill defined terms, so hopefully you would be happy with it if you read it in its full length. There is no such thing as "full stop period" qubits, hence the confusion.

>There is no such thing as "full stop period" qubits, hence the confusion.

of course there is - this is just more of exactly the same word play lol

>In 1995, Ben Schumacher provided an analogue to Shannon’s noiseless coding theorem, and in the process defined the ‘quantum bit’ or ‘qubit’ as a tangible physical resource

...

> For our elementary coding system we choose the two-level spin system, which we will call a "quantum bit" or qubit.

you can talk about surface plasmons or transmons or josephson junctions or whatever you want but the definition is always physical not logical (that's an abstraction!).

>I am not sure why you are so angry

I'm not angry - I already said what I am and that's tickled/mirthful but also bemused by the consistent evasion by the QC community to talk about physical reality.

So I think the truth is I'm not angry but you're defensive because a 100 qubit QC is useless, and moreso a 1000 qubit, even 100,000 qubit QC would be as well. But that's quite inconvenient for a research community that's publishing papers and submitting grant proposals.

You are making up claims about what people say and then get angry/tickled/mirthful about those made up claims. All throughout this thread people working in the field explicitly say that you can not use this device for simulating chemistry or factoring numbers and admit that you need at least about a million physical qubits (about a thousand logical qubits) to start doing these things reliably. How can you then claim that we are trying to hide it because it is "inconvenient"? This is the big thing to solve in our field, mentioned in every single research proposal, not some hidden secret we are ashamed of. And your refusal to distinguish physical and logical qubits is just silly and leading to your confusion.

>And your refusal to distinguish physical and logical qubits is just silly and leading to your confusion.

...bruh i'm a phd student whose work supports SQMS at fermilab (not in physics but cs). i'm not confused about absolutely any of the terms or definitions. hint: you're not the only QC researcher in the room at all times.

>You are making up claims about what people say and then get angry/tickled/mirthful about those made up claims.

i'm not making anything up - this comment

https://news.ycombinator.com/item?id=29245025

doesn't say absolutely anything about error correction and just vaguely alludes to coherent qubits being somehow different from physical qubits. like are you kidding me claiming that you're being transparent while reporting 100 anything without immediately revealing that it's actually 100k? somehow in your mind 3 orders of magnitude isn't a big deal when communicating relevancy/value/merit?

for a farcical analogy: can you imagine me reporting 100 dead corps and then come to find out i'm talking about 100 corporations being massacred, each corporation employing 1000 people.

there is no other academic discipline that plays this slight-of-hand. i'll give you another analogy that should be near and dear to your heart and will illustrate the point very precisely: can you imagine daniel simon saying he proved separation of BQP and BPP and not immediately (in the same sentence) revealing that it was oracle separation?

Again, read that series of comments again. I am saying I would be excited to work with a device with 100 physical qubits, not 100k. You are making up the least charitable possible interpretation of an offhand comment and getting angry at your imagination.

lololol you keep dancing around the issue - now we're on to comparing "excited" to "useful"; the question wasn't what you're excited to work with (i'm excited to work with 5 qubits through qiskit). the question, plain as day:

>what's the smallest useful (as in, 'non-toy', or maybe 'worth buying time on') quantum computer?

very obviously this person isn't asking about useful for writing papers...

>You are making up the least charitable possible interpretation of an offhand comment

nothing imagined here. just english. sorry.

> and getting angry at your imagination.

lol you keep insisting i'm angry. i mean if a reviewer reviews your submission and calls you out for inflating numbers i guess they're angry too? oh well

You are splitting hairs and picking arbitrary ways to interpret what I am saying. I would be excited, it would be useful, it would be great to work with a 100 physical qubits, so that we can test control schemes, small error correction procedures, maybe some entanglement purificant circuits, etc. You are willfully misinterpreting at this point.

no you are willfully misinterpreting this question posed by a layman

>what's the smallest useful (as in, 'non-toy', or maybe 'worth buying time on') quantum computer?

this question is not about papers or research - it is about value for problems/questions outside QC. simple as that. the answer to that question is ~100,000 physical qubits not 100.

again this whole exchange with you just further reaffirms that there's a very very strong reality distortion field around this entire area of academia.

And if you actually read my answer you see that I agree and in the first paragraph say that you would need around 1M physical qubits for anything practical!? The second paragraph specifically says it is about researchers in the field trying to push the field forward, playing with 100 physical qubits (yes, that usually involves writing papers).

What near term applications would be possible with just a 100 long-lived qubits?

With long-lived ones, gosh, a ton. When you hear a researcher talk about "logical qubit" or less formally "long-lived qubit" they mean a reliable abstract computational component. When we talk about "physical qubits" we mean the unreliable real implementations like the one from this article. A rough rule of thumb is that you need a 1000 physical qubits to make one logical qubit.

With 100 logical qubits (i.e. 100k physical qubits), you can start thinking about running chemistry simulations on the edge of what is possible with classical supercomputers. That is what I am excited about. There are also optimization problems, and some pretentious claims about quantum machine learning, which I am certain would be fun, but I am not as excited about.

With 100 physical qubits, you can start testing non-trivial control schemes, circuit compilations, error correction methods, and many other building blocks.

For example routing algorithms used to change railroad traffic in case of a problem with a junction or track.

Is it accurate to say the primary use of this is likely to design further quantum computers?

Yeah the R&D that goes into this is mainly part of the longer term effort to make increasingly large quantum computers.

In case you meant it the other way: the number of qubits here is still far too small for any real world application, including for simulations that would help design larger chips.

In a way, yes. I usually call such devices "technology demonstrators". But I am certain there would be researchers offended by such a trivialization of their work.

Kinda depends on what you want to use it for. If you want to simulate a physical system then you need at least as many qubits as the system has (and an understanding that such a simulation will have terrible noise). If you want to do something “useful” like run Shor’s algorithm to factor large encryption keys then it would take millions of qubits and quantum error correction.

To me the significance of this kind of increase in number of qubits is that many detractors of quantum computing had argued we’d never even reach this point, so I am slightly more optimistic that we’ll eventually reach the scale required for reliable abstract computations.

Why 433 and 1121? In classical computers, everyone knows why a lot of things are done in powers of 2. Is there a different base number useful for quantum computation that I don't know about (for example, is this an integer multiple of a specific sin/cos value)?

Not really. For this particular device they are laying out the physical qubits in some geometric lattice constrained by where they can put traces for microwave inductors and capacitors, and it just happens that this is the convenient size at which to try to build the lattice.

It depends on your use case. By some measure, the Google collaboration that demonstrated a time crystal was a practical quantum advantage in the space of condensed matter research. But in that space, practical means validating theory with experiment. That's a much lower bar for a quantum advantage than, say, showing that you've implemented a quantum algorithm that can, with high statistical likelihood (e.g. an error rate of 1/10e5), provide fleet routing solutions for a logistics company that are 10% more efficient than any known classical routing algorithm.

A real random number generator, this is basically the "hello world" on a quantum computer.

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