They finally did it. After years—no, decades—of declaring their hopes and dreams with hardly any practical results, researchers in the quantum computing community have delivered on a promise. Or have they?
Last week, news leaked that researchers at Google and other institutions had solved a problem on a quantum computer 1 billion times faster than a classical computer. Google did not respond to a request for comment, but according to a draft manuscript describing the experiment, they have realized “quantum supremacy,” an achievement that “heralds the advent of a much-anticipated computing paradigm.”
The reactions from the rest of the quantum community, however, have been downright contradictory. In an email, physicist John Preskill of Caltech calls the work a “truly impressive achievement in experimental physics.” Cautioning that the leaked manuscript is just a draft, mathematician Ashley Montanaro of the University of Bristol says this is a “genuinely exciting moment,” and also that the work is “not of practical relevance.” Dario Gil, the director of research at IBM, disputes the notion of quantum supremacy itself, calling the term “misleading” in a statement to WIRED.
So which one is true? Did the Google team just clear a steep technological threshold, or has it performed a largely useless experiment dressed up in clever branding? The truth lies somewhere in between, and captures the competing tensions at play in the quantum computing world.
On one hand, these researchers managed to execute an extremely complex experiment built on painstaking mathematical proofs and years of hardware development—an undeniable achievement. Yet the experiment brings them no closer to the money-making applications that the quantum community has promised, where the computer’s unique number-crunching capabilities will reveal new molecules for better batteries, drugs, and more, at speeds that would put normal computers to shame. It’s both a major victory and a bit of a letdown.
To understand this, let’s unpack what the Google paper actually describes. The researchers ran the experiment on a quantum computing chip named Sycamore, which contains 54 tiny objects called qubits that they can program to represent the digit 0, 1, or a weighted combination of both, called a superposition. The researchers apply voltage pulses and microwaves in various sequences to the qubits, changing their values according to rules set by quantum mechanics. String several such pulse sequences together, and you’ve written an algorithm for a quantum computer.
In the quantum supremacy experiment, Google’s scientists designed some pulse sequences that essentially turn their computer into a random number generator. They then made the quantum computer spit out millions of numbers. Though the numbers look random, they are meant to still fall into a pattern prescribed by Google’s algorithm. So they next checked to see if the numbers obeyed that distribution. They did.
On its own, this task is basically an excuse for a quantum computer and a supercomputer to race each other. Its practical implications are minimal. But this is the first race a quantum computer seems to have won. The supercomputer could not verify that the numbers obeyed the distribution in time.
The scientific effort behind this experiment began more than a decade ago, with researchers outside Google. In 2004, Barbara Terhal and David DiVincenzo, then at IBM, developed the initial mathematical evidence that qubits might compute this specific task faster than classical bits. It wasn’t clear anyone would ever try to perform the task; capable quantum computers didn’t even exist yet.