Google and IBM Fight on Quantum Supremacy

#### 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 super computer 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*. So Google’s claims are official now, and can be examined in the way any new science claim should be examined: sceptically until all the doubts are answered.

We have earlier covered what is quantum computing in these columns, and will move on to the key issue of quantum supremacy and what it really means.

IBM’s paper published around the same time as Google’s *Nature* paper, showed that Google was wrong in claiming that IBM’s 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.

Yes, Sycamore still beat Summit on this specific problem by solving it *1100 times faster,* and not *157 million times* faster as Google had claimed. But 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.

Regarding IBM’s claim that quantum supremacy has yet to be achieved, Scott Aaronsen, a leading quantum computing scientist, wrote that 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 super computer the size of a city!

Why does Summit have to increase at this rate to match Sycamore’s extra qubits? For the demonstration of quantum supremacy, Google chose the simulation of quantum circuits. The *resources* – disk space, memory, computing power – requited to solve this problem in reasonable time by classical computers increases *exponentially* with size of the problem. For quantum computers, adding qubits linearly – meaning simply adding more qubits – increases its *computing capacity exponentially*. Just extra 7 qubits of Sycamore needs IBM to increase the size of Summit 33 times; 17-qubit increase of Sycamore, needs Summit to increase 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.

Therefore, we have to give Google the “victory” here, not because IBM is wrong, but the *principle* of quantum supremacy, that a quantum computer can really beat a conventional computer in computing a certain class of problems in reasonable time has been established. The actual demonstration – a more precise definition of reasonable time and its actual demonstration – is only of academic value. If a 53 qubit can solve the problem, but with IBM’s Summit still in the race, even if much slower, it is only a matter of time before we see it truly beaten.

Of course, there are other ways that this particular test could fail. A new algorithm can be discovered which solves this problem faster, starting a fresh race. But the principle is not a specific race but the way quantum computing will scale in solving a certain class of problems that classical or conventional computers can’t.

The key issue in creating viable quantum computers should not be confused with a race between classical computers, and the new kid on the block, the quantum computers. If we see the race as between two classes of computers in solving a specific problem, we are missing the big picture. It is simply that for classical computers, solution time for a certain class of problems increases exponentially with the size of the problem, and beyond a certain size, we just can’t solve them in any reasonable time. Quantum computers have the potential for solving such problems in a reasonable time, therefore opening the door for computing such problems.

Are there such problems and will they yield worthwhile technological applications? The first problem chosen, computing the future states of quantum circuits were not chosen for any practical application. It was simply chosen to showcase quantum supremacy, defined as a quantum computer solving a problem that a classical computer cannot solve in reasonable time. Recently, a Chinese team led by Jianwei Pan, has a paper that shows another problem, a Boson Sampling experiment with 20 photons can also be a pathway to show quantum supremacy. These problems are constructed not for showing real world applications, but simply that quantum computing works can *potentially* solve real world problems. Provided we find the right problem.

The question is what are the class of problems that can use quantum computers?

The first class of problems are the ones for which Feynman had postulated the quantum computers, a simulation of the quantum world. Why do we need such simulations, as after all, we live in the macro-world in which quantum effects are not visible to us? The right word is *visible to us*, but they are all around us in different ways.

It is now clear using classical computers, that we cannot simulate for instance protein folding, as it involves the quantum world intersecting the macro-world. A quantum computer could simulate the probability of how many possible ways it could fold. This would allow us to build not only new materials but also medicines known as biologics and work out what are their actual medical pathways. So, new complex chemicals could be a field of discovery or new materials with complex molecules. Or real life problems which involve many combinations that again cannot be solved in reasonable time. The business world – IBM, Google – is gung-ho on the possible use of quantum computers for such applications and that is why they are all investing in it big time. So are nation states, notably the US and China.

But supposing quantum computers do not lead to any commercial benefits, should we then abandon quantum computing? If it is useful only for simulating quantum mechanics and understanding that world better? Did we build the Hadron Collider investing 13.25 billion dollars and an annual running cost of 1 billion dollars only because we thought we will have discoveries that will have commercial value? Or should society invest in knowing the fundamental properties of space and time including that of the quantum world? Even if the quantum computers only give us a window to the quantum world, the benefits would be knowledge. What is the price of this knowledge?