Recently, I gave a couple of perspective talks on quantum advantage, one at the annual retreat of the CIQC and one at a recent KITP programme. I started off by polling the audience on who believed quantum advantage had been achieved. Just this one, simple question.No, quantum supremacy has not been achieved, if most experts are not convinced. The whole was to do a public demonstration that convinces everyone that quantum computers are possible.The audience was mostly experimental and theoretical physicists with a few CS theory folks sprinkled in. I was sure that these audiences would be overwhelmingly convinced of the successful demonstration of quantum advantage. After all, more than half a decade has passed since ...
I could not have been more wrong.
In both talks, less than half of the people in the audience thought that quantum advantage had been achieved.
Monday, January 19, 2026
Has quantum advantage been achieved?
Dominik Hangleiter writes on his blog:
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The only thing confirmed about 'quantum computing' is that it is expensive. Everything else is pretty uncertain.
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ReplyDeleteThe problem of reliably controlling a large number of quantum particles is really hard.
ReplyDeleteA somewhat relevant paper is here: https://arxiv.org/abs/0805.0771 .
Of course, it talks about PMTs. The physics for qubits is in many ways different. But certain principles do remain in common to both.
Yet, a relevant consideration is this: PMT's are designed to produce large nonlinear cascades, but in a controlled way (within a narrow window around the cusp of the dynamical regimes). This paper shows why a very widespread expectation regarding such a control does turn out to be too naive.
In contrast, qubits are designed to suppress all nonlinear cascades and still remain controllable.
So, the point becomes: Can the same / similar factors which affect PMTs (given their design goal) also become relevant for qubits (given their design and goals), and whether error-correcting ``codes'' might be enough.
But still, speaking overall, the general imagination that the lower the temperature you can get to, the ``cooler'' it is to realize control over qubits, might be too naive. That was my point.
And, another point: No, this still does not prove that QC's are in principle impossible. But it does help shed some light on how hard the problem actually is.
I might write a bit more at my blog, hopefully soon enough. But no promises. And, yes, people should lower their expectations anyway. No one pays me for my research anyway. Or, for that matter, no one even sponsors me for uploads to arXiv. What moral or other rights do people then have? or means? mere psychological inducements and/or attacks? LOL!... Something worse? possible? in what way?...
--Ajit