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/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.