While System One went online in January 20 19, a set of starter computers called IBM Q Experience has been available online for the last three years ; anyone can log on and write and run programs on them. To date, Dr. Gil said, some 130,000 people have used it, running 17 million experiments and publishing some 200 papers. And there were more quantum devices, behind other doors, operated by scientists trying to learn how to speak nature’s exotic subatomic language. “I’m convinced there are more quantum computers working here than the rest of the world combined, in this building,” Dr. Gil said.

Quantum supremacy, maybe

Mathematicians are still debating what might be accomplished with all this quantum power when it finally grows up. Ordinary computers are good for solving “easy” problems — questions that can be answered in a reasonable amount of time, like navigating the rings of Saturn or predicting the path of a hurricane.

Then there are “hard” problems, whose solutions are difficult to find but, once identified, are easy to verify. Among them is the factoring of large numbers. Many modern encryption schemes, like the widely used RSA cryptographic algorithm, rely on the inability to factor such numbers in a reasonable amount of time.

In 1994 Peter Shor, then at Bell Labs and now at M.I.T., devised an algorithm that a quantum computer ( a still-hypothetical device at th e time) could use to factor big numbers and thus break most cybersecurity codes now in common use .

In 2012 Dr. Preskill, the Caltech physicist, invented the term “quantum supremacy” to describe the potential of quantum computers to drastically outperform classical ones.

That is what a Google team has been trying to do with a quantum computer called Sycamore. The calculation they are tackling is highly specialized and technical, designed mostly to show that quantum supremacy is possible.

Success would be an inflection point in the march of human knowledge, a baby step toward a radically different future, like the first Wright Brothers flight. But it’s only one step on a long road.