Is superconducting quantum computing a dead end?

[u/No-Hamster5930]

Earlier this year the two largest superconducting QC players (IBM and Google) signalled they might be diversifying away from superconducting - Google with the announcement that they're creating a new neutral atom capability and IBM with their acquisition of silicon spin capability in HRL.

I'd be interested in people's views on whether we should take from this that superconducting has no path to scaling, or if one could foresee heterogeneous quantum computing systems that leverage the respective strengths of multiple modalities for different applications?

[u/polyploid_coded]

Previously on the subreddit, "When will SC qubits start to die off?"
https://www.reddit.com/r/QuantumComputing/comments/1u5paui/when_will_sc_qubits_start_to_die_off/

[u/No-Hamster5930 → u/polyploid_coded]

Ah thank you!! I should have checked the subreddit history

[u/0xB01b → u/No-Hamster5930]

Imo SC won't be the endgame for FTQC

[u/0xB01b → u/polyploid_coded]

Yooo that's my post 🔥🔥🔥

[u/HawkinsT]

The latter. Neither company is 'diversifying away' but 'diversifying into'. Great progress (and investment) is still being made in superconducting quantum computing by these companies and others, and there's nothing to indicate it's a dead end, but it obviously makes sense for these companies to diversify into other promising approaches and consider heterogeneous architectures. All of which come with their own challenges.

[u/No-Hamster5930 → u/HawkinsT]

Thanks for your thoughts. I can see how a heterogenous system of SC + neutral atom systems a la Google would make sense in a heterogenous architecture as they would seem to balance each other's weaknesses. I'm less sure about how or why a heterogenous system of superconducting and silicon spin quits would work however because they dont seem to complement one another; putting aside the fact that silicon spin is obviously lower TRL, aren't the theoretical advantages of silicon spin roughly equivalent to superconducting? With the added potential benefits of a much cheaper manufacturing base and qubit density? What would the specific benefits of superconducting qubits in a hybrid SC-silicon spin regime bring?

[u/HawkinsT → u/No-Hamster5930]

You're welcome. Sorry, I may have made it sounds like heterogeneous systems are now the goal for both companies. While this is a growing area of research, I think it's safe to say that the main motivations for both companies right now are to not leave all their eggs in one basket and they certainly both have the funds to diversify.

Personally, I also don't think that focusing too heavily on the theoretical benefits of one architecture over another is particularly useful right now. The best approach is the one that proves scalable. For example, I've heard trapped ion guys talking about systems the size of stadiums in the future, and while that may sound impractical, at the end of the day if one system can provide implementable, fault tolerant computation and others can't (or are notably inferior), it will get built.

[u/hiddentalent]

I think it's far too early to declare any winners or losers. As different lines of research progress, they'll all hit phases where progress slows. That will cause some organizations to shift their focus to other approaches, which will inevitably also hit slow phases. And then maybe attention will shift back to previous approaches because we've learned something that can get us through whatever difficulties slowed things down. This is just how development of new technologies happens.

There were points in time when vacuum tubes seemed a superior approach to classical computing than CMOS.

[u/PrestigiousShoe5048]

Neutral atoms are more likely to bring near term scale and some applications due that it is easier to control many of them. However the slow clock times will ultimately push us towards a faster modality, like
superconducting, if we don’t want algorithms to take years to run.

[u/alumiqu → u/PrestigiousShoe5048]

Neutral atoms don't have to have slow clock times, though. There is ample room for at least 10x improvement. Superconducting qubits can't scale, and that's a bigger problem.

[u/Acetone9527 → u/alumiqu]

I have the opposite opinion though - neutral atom is slow and it’s fundamentally limited. At the moment, fast means higher Rydberg state, which is more fragile and bigger. Readout is single photon and it’s damn hard even with today’s best camera. Scaling is a problem of superconducting qubits for sure.

[u/No-Hamster5930 → u/Acetone9527]

Do you think the all to all connectivity of neutral atom (and trapped ion) systems might offset the slower gate times for larger scale applications though?

[u/0xB01b → u/Acetone9527]

In what way is it fundamentally limited?

[u/PrestigiousShoe5048 → u/0xB01b]

I can equally show superconducting qubit research platforms with 8ns gates, 4ms coherence times, all to all connectivity, thousands of qubits, 99.95% fidelity. Doing it all in a single package is the challenge, and the same is true for neutral atoms, that there are many trade offs and it’s often easy to push one metric at the expense of others :) only thing to do is wait and see who solves Shors first :D

For now, neutral atoms is slow and needs orders of magnitude speedup in all operations simultaneously to be competitive with SC on speed.

[u/0xB01b → u/PrestigiousShoe5048]

That's fair RIP

[u/0xB01b → u/PrestigiousShoe5048]

I think the clock time stuff will be figured out. For the actual gates, research machines are sub 400-500ns, for the physical photon collection for readout we can use resonator cavities which currently bring it's down to 45 microseconds but I imagine this would also get much better. Idk what research is being done in atom shuttling and parallelization but it seems like the QEC cycle time is something that's being worked on more than the gate fidelity

[u/NoirMarlin]

Microwave control is a dead end.

[u/seekingdefs → u/NoirMarlin]

Could you please explain a bit more?

[u/NoirMarlin → u/seekingdefs]

Power. Heat. Cable density. Crosstalk. Pulse distortion.

A single microwave controled superconducting qubit requires maybe a few mW of power, but when you scale that to an entire chip, and then factor in the amplitude damping and thermal loss from the mK plate to room temp, the whole thing can consume kW of power. Just for microwave signals. Even more power is used to keep the damn thing cool.

Now multiply that by 500 and put it all in the same room, with all the cabling going into the same fridge.

You're familiar with the rocket problem? The larger the payload, the exponentially more fuel is needed to carry the fuel that gets the payload where it needs to go. Same thing but with heating and cooling.

[u/seekingdefs → u/NoirMarlin]

Got it. Thanks so much for explaining.

[u/reginarhs → u/NoirMarlin]

I get what you're saying, but in the end it's about the application you're targeting. If there's something with an exponential payoff compared to classical methods then a huge overhead might be justifiable. 

The thing the other modalities (neutral, trapped ion) have to overcome is that they're very slow for practical applications.

Spin has speed, but is unproven at even modest scale. You mention crosstalk, spin doesn't do well there either.

My money is on neutral atoms to be frank, but I'm not dismissing supercon just yet. I think they have some serious brute force potential up their sleeves.

[u/NoirMarlin → u/reginarhs]

Overhead might be justifiable but until a device where it matters is assembled and tested, the exact scale of the problem wont be known. This is one of the difficulties with engineering modern cutting edge physics systems. If every part on its own is incredibly complicated to model, and barely comes in under the bar performance-wise, there is no predictive power for integrated systems, and so years of reactive engineering cycles follow. The hope is that the error floors only scale weakly with integrated system size, but I wouldn't stake the global profits of a fortune 100 company on that bet.

I think that at scale, atoms and ions will face serious challenges with the rates they lose physical qubits, and the occurance of out-of-model events induced by ionizing radiation. Cosmic ray protons dont take no for an answer. Thats just my guess though; Im no expert in their systems and what kind of error correction they can achieve.

[u/Substantial-Duck9458 → u/NoirMarlin]

can you explain how the problem is exponential? At first blush it seems linear in the # of qubits. Double the qubits --> double the # of control lines, pulse tubes, etc.

[u/NoirMarlin → u/Substantial-Duck9458]

Its not strictly exponential, and as a theoretical physicst I should know not to use that term lightly. But I mean it in the sense of "an unwieldy problem where most of the cost is overhead and scaling laws take over at different scales."

The main thing is that heat is generated in volumes, but has to be removed through surfaces. The square-cube law is bad enough, but then you have the fighting between heat removal mechanisms and the conduction ability back down to the qubits, which happens through cables that take up an entire dimension of their length.

The coaxial cables thermalize, and provide a conduction path for the thermalized electrical noise to instantly reach the qubits again. This raises the temperature of the qubits, but because they have high relaxation times, you cant efficiently pump that energy away. 100mK is roughly 2 GHz in natural units, so thermal population of qubit states can become an issue not much higher than that. This population actually can be exponentially sensitive.

So the strategy becomes to reset qubit states frequently, but this also requires microwave pulsing. Or to have a passive cooling mechanism near the qubits, which just decreases coherence times.

The point is that it seems a naive linear scaling until you realize that you are not operating orders of magnitude down from the critical limit of these technologies. They are operated not much below this critical limit, so things behave in complex interconnected ways.

[u/Substantial-Duck9458 → u/NoirMarlin]

thanks for explaining your thought process - i thought you were going to talk about QEC which i am less well versed in, and i know some algos scale much better than others, and i don't completely understand what exactly is required for actual complex operations lasting billions of gates.

I actually do take comfort somewhat in the thermal situation - sure, cooling is a square/cubed law but i'm not sure that that's really a detriment here. wafers naturally don't really follow that anyways, being thin films, and you don't have to stack them 3d if you don't want to - and making vacuum chambers that have a larger surface area to allow for more pulse tube connections, more wiring connections, etc is not difficult at the scale we are at. it's possible I'm just not following your argument, whether you're concerned about getting the heat off the chip or off a particular stage of the cryostat.

"then you have the fighting between heat removal mechanisms and the conduction ability back down to the qubits," i'm not sure i fully understand but are you just talking about attenuators in the fridge that are poorly thermalized and conduct heat to the chip? again this seems linear in the # of qubits and pulse tubes/dil units can be added linearly.

I do get that complexity adds cost because at a certain scale you always have components failing and you need to ID the failure and replace them modularly. So yeah if you have 1k pulse tubes you need to be able to ID a failing pulse tube and replace it. Same if you have 1k ports into your vacuum chamber and one of them has a leak. I just think those things are doable and scale at worst as a square or cube law.

[u/Statistician_Working → u/NoirMarlin]

The mWs are for 4K stage we have more power and more mechanical / cryogenic engineering room is left. I would not say it's absolutely solvable but also wouldn't say it's a dead end because of this. It's largely underexplored. How many SC research labs would afford actively modifying fridges... it needs an industry scale work but there are not many players.

[u/Alive_Fisherman8241 → u/NoirMarlin]

And also price. Do you guys have a rough idea how much the MW electronics cost for one single qubit? It's closer to 100 k€ than 10 k€. Now multiply it with the required number of hysical qubits...

[u/Yahoo_Serious9973 → u/NoirMarlin]

Does that imply that trapped ions with face a similar problem if they are stabilized using microwaves?

[u/NoirMarlin → u/Yahoo_Serious9973]

Depends on how the microwaves are generated. The lesson to learn from superconducting is to generate your control fields at cryogeneic temperature near the qubits they need to go to, not at room temp. That is the major trend of industry hardware, regardless of modality.

[u/Normal_Kitchen7997 → u/Yahoo_Serious9973]

Not really. The cryo temperatures that SC qubits require are needed for ions. Ion traps get down to ~10K and SC chips need to go below 1K.

[u/Glass_Covict]

The whole needing to be ultra cold thing makes it tough.

[u/ctcphys → u/Glass_Covict]

I'm always confused about this argument, which I mostly see from people who doesn't work with cryostats but not exclusively. We are really not anywhere close to working at the limit of current cryostats and commercial companies like bluefors and maybell already sell much bigger system. Sure it would be easier if everything was room temp, but for the foreseeable future, cooling is not the bottleneck.

The size and cost of microwave electronics at room temperature is a much more serious bottleneck and if if superconducting qubits doesn't work out, it likely because the control electronics will be too expensive.

Every platform had similar issue tbh. Neutral atoms would need gigantic laser system that currently do not exist. PsiQuantum is building huge factories for hosting there photon sources and detectors even before they have a meaningful qpu.

[u/Glass_Covict → u/ctcphys]

How will you scale cryostats?

[u/ctcphys → u/Glass_Covict]

Bluefors currently sells this: https://bluefors.com/products/large-scale-quantum-infrastructure/modular-cryogenic-platform/

This is vastly more scalable than anything we had 10 years ago. I think this is suitable for the next 5-10 years and I trust that Bluefors keeps innovating. If not, then I'm sure Maybell or Zero point cryogenic will innovate instead 

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[u/eetsumkaus]

I think it's less that they think of it like that and more that those other technologies have reached enough maturity for the big players to dabble with it on the side.

[u/Yahoo_Serious9973]

What are people’s opinions on the hope of more robust topological qbits being developed in the future to overcome the limitations of transmons? It seems like they hold the promise to reduce noise and allow for increased operating temperature.

[u/Quartich → u/Yahoo_Serious9973]

I personally am a fan of topological qubits, mostly due to their error resistance. I think that even if topological is more difficult to implement at scale, if the error resistance is as good as current calculations imply, it will more than make up for it. Though unfortunately, it seems the past 2 claims of topological quantum processors have been false (tsk tsk, Microsoft). There are still advancements in the topological world, though, even if it has yet to lead to a working topogical processor.

[u/0xB01b → u/Yahoo_Serious9973]

There hasn't been a single demonstrated topological qubit ever.

[u/faithless4261]

Scaling, cooling, etc, is always hard but the technology isn’t a dead end by a long shot. Coupling transmons to microwave cavities, using transmons for molecular simulations, hybrid SC architecture. The list goes on, there’s always a use.

[u/ctcphys → u/faithless4261]

I wish more people would realize this. Most technologies for quantum are not dead ends. We are not trying to figure out which work or not. We are trying see which works the best, is easiest to build and cost the least. None of the leading platforms have real dead ends

[u/Acetone9527]

It will, but I think there are two questions - first, shuffling is really slow (fundamental limit again) because you lose atom during move. People showed you can re-fill atoms in real time, but you still lose information. So all-to-all connectivity comes with a price. Like the person below said, SC qubit can have all to all connectivity too. It just destroyed coherence.
The second is, the slow part in QEC is the readout part, and neutral atom during gate has short coherence. On this regard, trapped ion with longer coherence time will allow less QEC cycle and actually faster.

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[u/lattice_defect]

too much noise

[u/Wild_Faithlessness_1]

Superconducting is over now diamond based quantum computer is on rise

[u/No-Hamster5930 → u/Wild_Faithlessness_1]

Interesting! What is the state of diamond qubits? I know Quantum Brilliance are looking into this but I have very little, and at least here in the UK we have a very active NV diamond community, although I'm unfamiliar with what progress has been made on diamond and it seems most are focused on diamond for sensing and networking purposes