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

The tech giant announced it had reached a long-anticipated milestone known as “quantum supremacy” — a watershed moment in which a quantum computer executes a calculation that no ordinary computer can match. In a new paper in Nature, Google described just such a feat performed on their state-of-the-art quantum machine, code named “Sycamore.” While quantum computers are not yet at a point where they can do useful things, this result demonstrates that they have an inherent advantage over ordinary computers for some tasks.

Quantum computers have been under development for decades. While ordinary, or classical, computers perform calculations using bits — strings of 1s and 0s — quantum computers encode information using quantum bits, or qubits, that behave according to the strange rules of quantum mechanics. Quantum computers aim to harness those features to rapidly perform calculations far beyond the capacity of any ordinary computer. But for years, quantum computers struggled to match the computing power of a handheld calculator.

The bad news – for the science fiction enthusiasts – is that it is not going to replace our computers but will be useful for a special class of problems. Its construction requires conditions such as super low temperatures that can be created only in a special environment. We are not going to wear it on our sleeves or use it on our cell phones. At least not yet, and not with today’s physics! And our encryption algorithms on which all our internet protocols and world’s financial transactions are based are safe, at least for now.

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.

In their Nature paper, Google claims that their Sycamore processor took 200 seconds to perform a calculation that the world’s best supercomputer — which happens to be IBM’s Summit machine — would need 10,000 years to match. That’s not a practical time frame. But IBM now argues that Summit, which fills an area the size of two basketball courts at the Oak Ridge National Laboratory in Tennessee, could perform the calculation in 2.5 days.

Google stands by their 10,000 year estimate, though several computer experts interviewed for this article said IBM is probably right on that point. “IBM’s claim looks plausible to me,” emailed Scott Aaronson of the University of Texas, Austin.

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

So why do the machines based on classical physics not work on quantum phenomena? Simply put, the calculations of such systems would grow exponentially with the size of the system, or the increase in time horizon for which the end state is being computed. In any case, the future states of the quantum world are probability distributions, and are captured better by quantum computers which give their results also as probability distributions.

So what is the difference between computers built on quantum principles and those on classical physics? All our computers in everyday use – classical computers in this language – the information in the system exists only in binary form, the smallest bit of information that exists is either a 0 or a 1 (False=0, True=1). The quantum computer has quantum bits – qubits – that exist in many different states simultaneously, using the quantum phenomenon of superposition. The final value of this superposition can be found only when it is measured, when it “collapses” to either a 0 or a 1. When that happens the qubit life is essentially over, it can no longer be used in further calculations.

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.