by PRABIR PURKAYASTHA
Google’s quantum supremacy claim has now been disputed by its close competitor IBM. Not because Google’s Sycamore quantum computer’s calculations are wrong, but because Google had underestimated what IBM’s Summit, the most powerful supercomputer in the world, could do. Meanwhile, Google’s paper, which had accidentally been leaked by a NASA researcher, has now been published in the prestigious science journal Nature. Google’s claims are official now, and therefore can be examined in the way any new science claim should be examined: skeptically until all the doubts are addressed.
Previously, I have covered what quantum computing is, and in this article, I will move on to the key issue of quantum supremacy, the claim that IBM has challenged and what it really means. IBM concedes that Google has achieved an important milestone, but does not accept that it has achieved quantum supremacy.
IBM refuted Google’s claim around the same time as Google’s Nature paper was published. Google had claimed that IBM’s supercomputer, Summit, would take 10,000 years to solve the problem Google’s Sycamore had solved in a mere 200 seconds. IBM showed that Summit, with clever programming and using its huge disk space, could actually solve the problem in only 2.5 days. Sycamore still beat Summit on this specific problem by solving it 1,100 times faster, but not 157 million times faster, as Google had claimed. According to IBM, this does not establish quantum supremacy as that requires solving a problem a conventional computer cannot solve in a reasonable amount of time. Two and a half days is reasonable, therefore — according to IBM — quantum supremacy is yet to be attained.
The original definition of quantum supremacy was given by John Preskill, on which he now has second thoughts. Recently he wrote, “… supremacy, through its association with white supremacy, evokes a repugnant political stance. The other reason is that the word exacerbates the already overhyped reporting on the status of quantum technology.”
Regarding IBM’s claim that quantum supremacy has not yet been achieved, Scott Aaronson, a leading quantum computing scientist, wrotethat though Google should have foreseen what IBM has done, it does not invalidate Google’s claim. The key issue is not that Summit had a special way to solve the specific quantum problem Google had chosen, but that Summit cannot scale: if Google’s Sycamore goes from 53 to 60 qubits, IBM will require 33 Summits; if to 70 Qubits, a supercomputer the size of a city!
Why does Summit have to increase at this rate to match Sycamore’s extra qubits? To demonstrate quantum supremacy, Google chose the simulation of quantum circuits, which is similar to generating a sequence of truly random numbers. Classical computers can produce numbers that appear to be random, but it is a matter of time before they will repeat the sequence.
The resources — disk space, memory, computing power — classical computers require to solve this problem, in a reasonable time, increase exponentially with the size of the problem. For quantum computers, adding qubits linearly — meaning, simply adding more qubits — increases computing capacity exponentially. Therefore, just 7 extra qubits of Sycamore means IBM needs to increase the size of Summit 33 times. A 17-qubit increase of Sycamore needs Summit to increase by thousands of times. This is the key difference between Summit and Sycamore. For each extra qubit, a conventional computer will have to scale its resources exponentially, and this is a losing game for the conventional computer.
We have to give Google the “victory” here, not because IBM is wrong, but because the principle of quantum supremacy, that a quantum computer can work as designed, solve a specific problem, and beat a conventional computer in computational time has been established. The actual demonstration—a more precise definition of reasonable time and its physical demonstration — is only of academic value. If 53 qubits can solve the problem, but with IBM’s Summit still in the race, even if much slower, it is just a matter of time before it is well and truly beaten.
Of course, there are other ways that this particular test could fail. A new algorithm can be discovered that solves this problem faster, starting a fresh race. But the principle here is not a specific race but the way quantum computing will scale in solving a certain class of problems that classical or conventional computers cannot.
For problems that do not increase exponentially with size, the classical computers work better, are way cheaper, and do not require near absolute zero temperatures that quantum computers require. In other words, classical computers will coexist with quantum computers and not follow typewriters and calculators to the technology graveyards.
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