Google's 49-Qubit Quantum Supremacy Deadline: What the Record Shows

John Martinis led Google's quantum computing group. He gave himself just a few months to reach a milestone in the history of computing. By the end of 2017, he said, his team would build a device that achieves "quantum supremacy," a calculation beyond the reach of any conventional computer.

"We think we're ready to do this experiment. It's something we can do now," Martinis told MIT Technology Review, in a piece published on 21 April 2017.

Six qubits, 49 qubits, and the gap between them

The numbers matter here. Google's newest chip at that point had six qubits, arranged in a two-by-three configuration, according to MIT Technology Review. Martinis said the layout was a test. It showed the company's technology still worked when qubits sat side by side, as they would in larger devices. The same chip tested a manufacturing approach in which the qubits and the conventional wiring that controls them are made on separate chips and later bump bonded together. Google's team had focused on that method since it was set up just over two years earlier, because extra control lines in a larger chip can interfere with how qubits function.

But the supremacy experiment needed a grid of 49 qubits, Martinis said, not six. Earlier Google results had used nine qubits arranged in a line. Designs for devices with 30 to 50 qubits were already in progress. He briefly showed images of the six-qubit chip at the IEEE TechIgnite conference in Burlingame, California, though the group had not formally disclosed technical details.

Google was not alone in the race. MIT Technology Review named Intel, Microsoft and IBM as competitors, along with startups. Simon Gustavsson, a principal investigator in a quantum computing research group at MIT, described Google as one of the leaders. "It's pretty comparable between Google and IBM," he said.

IBM's counter-announcement

On 10 November 2017, IBM said its scientists had built and measured a processor prototype with 50 quantum bits. Dario Gil, who led IBM's quantum computing and artificial intelligence research division, said it was the first time any company had built a quantum computer at that scale, according to the Associated Press, which reported the announcement via The Seattle Times.

Seth Lloyd, an MIT mechanical engineering professor not involved in IBM's research, said IBM likely still had glitches to work out, but called the 50-qubit announcement a sign of significant progress.

"Press releases always talk about quantum space race in number of qubits. It's more than just quantity, it's qubit quality."

That line came from Martinis, speaking at the first quantum computing for business conference, or Q2B, hosted by startup QCWare at NASA Ames. Gizmodo, which covered the December 2017 event, reported that Google was then fabricating its 49 or 50 qubit supremacy device and would begin testing in two weeks, not counting Christmas vacation delays.

Martinis also explained what the test would involve. Google's team would entangle its qubits into a quantum circuit and let the system evolve. The final measurement could take different values with different probabilities, and working out those outcomes is so complex that a classical computer has to simulate the quantum computer to do it at all, Gizmodo reported. That might take weeks to do what the quantum machine did in minutes. The problem is not useful for industry. It is a benchmark.

What the milestone was not

Expectations needed calibrating, and the sources in the record say so plainly. Chris Monroe, a professor at the University of Maryland and cofounder of the quantum computing startup IonQ, called the supremacy experiment "an academic milestone." "Afterward you still have to figure out how to make it more scalable and programmable," he told MIT Technology Review.

Martinis agreed that much remained to be done, but argued the experiment could become a benchmark for anyone claiming to have a working quantum computer. He also said the target had helped managers at Google, and cofounder Sergey Brin, appreciate that the technology was becoming real. "They all get it and are very excited about it," he said. "We're trying to get support within Google, and this experiment has been very good to get other engineers talking to us."

The timeline slipped. Google's claim of quantum supremacy arrived in September 2019, not 2017. Fortune reported that the company said its Sycamore processor, with 53 qubits, sampled a randomly generated circuit in about 200 seconds, while a state-of-the-art supercomputer would need approximately 10,000 years for the equivalent task. The researchers estimated the same experiment on a Google Cloud server would take 50 trillion hours; on the quantum processor it took 30 seconds. Fortune obtained a copy of the paper, which had been posted to NASA.gov and taken down; the Financial Times first reported the news. A Google spokesperson declined to confirm the paper's authenticity, and a source at Google suggested NASA had published it early, before peer review.

IBM pushed back on the framing. Dario Gil, head of IBM Research, told Fortune the experiment was a highly special case "laboratory experiment" with "no practical applications." "Quantum computers will never reign 'supreme' over classical computers, but will rather work in concert with them, since each have their unique strengths," he said. Jim Clarke, Intel Labs' director of quantum hardware, called the update "a notable mile marker" and said a commercially viable quantum computer would require many more R&D advancements. "While development is still at mile one of this marathon, we strongly believe in the potential of this technology," he said.

IBM had already begun arguing for a different yardstick. At the American Physical Society March Meeting in 2019, ZDNET reported, IBM outlined its highest Quantum Volume to date. Quantum Volume is a performance metric combining qubit count, connectivity and coherence time with gate and measurement errors, device cross talk and compiler efficiency. IBM said its Q System One, with a 20-qubit processor, produced a Quantum Volume of 16, double the then-current IBM Q at 8, and had some of the lowest error rates IBM had measured. The company said Quantum Volume would need to double every year to reach Quantum Advantage, the point where quantum applications deliver significant advantages over classical computers, within the next decade. IBM has doubled the power of its quantum computers annually since 2017.

All of this sits inside a field that, by the accounts in the record, is far from finished. At Q2B, Caltech theoretical physicist John Preskill described the current period as the NISQ era, for Noisy, Intermediate-Scale quantum computers. Preskill said progress towards a fault tolerant quantum computer must continue. Gizmodo noted that businesses would not be able to take advantage of quantum computing for quite some time, maybe decades.

Google's researchers were more optimistic in their 2019 paper. "Quantum speedup is achievable in a real-world system and is not precluded by any hidden physical laws," they wrote, according to Fortune. They predicted quantum computing power would grow at a double exponential rate, faster than the exponential rate that defined Moore's Law. And they added a caveat that has aged well: "We are only one creative algorithm away from valuable near-term applications."