What the Google vs. IBM debate over quantum supremacy means

Everyone loves a battle between two worthy champions where both combatants come out on top.

That’s what happened in the past two weeks between Google and International Business Machines in the fight over what the term “quantum supremacy” means. Both sides showed intriguing, valuable research.

In Google’s case, it’s about the physics of making a superior device. In the case of IBM, the company shows that “architecture,” the design of a traditional computer system, still has amazing potential to advance computing.

To recap, Google last week revealed in Nature magazine the results of its “Sycamore” superconducting computer chip, which was able to measure the output of a random number generator one million times in roughly three minutes versus what Google estimated would take 10,000 years to do using a conventional electronic or a “classical” computer. (Google also posted a blog item describing the work.)

A week before Google’s report was published, IBM issued its own report, based on a leaked version of Google’s research, stating that Google hadn’t achieved quantum supremacy. IBM claimed that, in theory, a supercomputer using conventional electronics could do the task not in 10,000 years but two and a half days.

What’s going on here? The debate is over what it means when you run an actual quantum computer, such as Sycamore, and compare it to a simulation of that quantum computer inside of a classical, electronic computer.

Quantum simulation software, such as Microsoft’s LIQUi|⟩ program, allows a traditional computer to represent a quantum computer in ordinary circuitry, by translating quantum mechanics into mathematical structures, known as matrices of complex numbers (numbers that incorporate both real and imaginary numbers).

With simulations, it’s possible to compare how long it takes real quantum circuits to produce a given computation, and how long the same computation takes a classical computer to reproduce, by running the matrix math that resembles the functions of the quantum circuit.

Google and IBM are both looking at such simulations, and they’re taking different views as to what the comparison means.

Google’s point is that Sycamore is a device that does the work it takes millions of conventional processors to simulate. As the authors state, when they simulated even a simplified version of the random number generator on the classical computer, it “takes one million [conventional computing] cores 130 seconds, corresponding to a million-fold speedup of the quantum processor relative to a single core.” Google ran its simulations on the Jülich supercomputer, in the German city of that name, and also on Google’s own cloud computing clusters.

So, Google is showing a better mousetrap, a device with physics that are simply superior to that of silicon circuits.

IBM, on the other hand, didn’t run any actual simulations. Instead, the company came up with a model on paper, a theoretical estimate for how long it would take to simulate Sycamore on the Summit supercomputer at Oak Ridge National Laboratories. In other words, IBM has put together a thought experiment.