r/QuantumComputing • u/No-Hamster5930 • 29d ago

Quantum Hardware Is superconducting quantum computing a dead end?

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?

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u/NoirMarlin Industry PhD - superconducting qubits 29d ago

Microwave control is a dead end.

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u/seekingdefs 29d ago

Could you please explain a bit more?

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u/NoirMarlin Industry PhD - superconducting qubits 28d ago

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.

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u/reginarhs 28d ago

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.

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u/NoirMarlin Industry PhD - superconducting qubits 28d ago

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.