Google’s claim to have demonstrated quantum supremacy—one of the earliest and most hotly anticipated milestones on the long road toward practical quantum computing—was supposed to make its official debut in a prestigious science journal. Instead, an early leak of the research paper has sparked a frenzy of media coverage and some misinformed speculation about when quantum computers will be ready to crack the world’s computer security algorithms.
The moment when quantum computing can seriously threaten to compromise the security of digital communications remains many years, if not decades, in the future. But the leaked draft of Google’s paper likely represents the first experimental proof of the long-held theoretical premise that quantum computers can outperform even the most powerful modern supercomputers on certain tasks, experts say. Such a demonstration of quantum supremacy is a long-awaited signpost showing researchers that they’re on the right path to the promised land of practical quantum computers.
“For those of us who work in quantum computing, the achievement of quantum supremacy is a huge and very welcome milestone,” says Scott Aaronson, a computer scientist and director of the Quantum Information Center at the University of Texas at Austin, who was not involved in Google’s research. “And it’s not a surprise—it’s something we all expected was coming in a matter of a couple of years at most.”
What Is Quantum Computing?
Quantum computing harnesses the rules of quantum physics that hold sway over some of the smallest particles in the universe in order to build devices very different from today’s “classical” computer chips used in smartphones and laptops. Instead of classical computing’s binary bits of information that can only exist in one of two basic states, a quantum computer relies on quantum bits (qubits) that can exist in many different possible states. It’s a bit like having a classical computing coin that can only go “heads” or “tails” versus a quantum computing marble that can roll around and take on many different positions relative to its “heads” or “tails” hemispheres.
Because each qubit can hold many different states of information, multiple qubits connected through quantum entanglement hold the promise of speedily performing complex computing operations that might take thousands or millions of years on modern supercomputers. To build such quantum computers, some research labs have been using lasers and electric fields to trap and manipulate atoms as individual qubits.
Other groups such as the Google AI Quantum Lab led by John Martinis at the University of California, Santa Barbara, have been experimenting with qubits made of loops of superconducting metal. It’s this approach that enabled Google and its research collaborators to demonstrate quantum supremacy based on a 54-qubit array laid out in a flat, rectangular arrangement—although one qubit turned out defective and reduced the number of working qubits to 53. (Google did not respond to a request for comment.)
“For the past year or two, we had a very good idea that it was going to be the Google group, because they were the ones who were really explicitly targeting this goal in all their work,” Aaronson says. “They are also on the forefront of building the hardware.”
Google’s Quantum Supremacy Experiment
Google’s experiment tested whether the company’s quantum computing device, named Sycamore, could correctly produce samples from a random quantum circuit—the equivalent of verifying the results from the quantum version of a random number generator. In this case, the quantum circuit consisted of a certain random sequence of single- and two-qubit logical operations, with up to 20 such operations (known as “gates”) randomly strung together.
The Sycamore quantum computing device sampled the random quantum circuit one million times in just three minutes and 20 seconds. When the team simulated the same quantum circuit on classical computers, it found that even the Summit supercomputer that is currently ranked as the most powerful in the world would require approximately 10,000 years to perform the same task.
“There are many in the classical computer community, who don't understand quantum theory, who have claimed that quantum computers are not more powerful than classical computers,” says Jonathan Dowling, a professor in theoretical physics and member of the Quantum Science and Technologies Group at Louisiana State University in Baton Rouge. “This experiment pokes a stick into their eyes.”
In a twist that even Google probably didn’t see coming, a draft of the paper describing the company’s quantum supremacy experiment leaked early when someone—possibly a research collaborator at the NASA Ames Research Center—uploaded the paper to the NASA Technical Reports Server. It might have sat there unnoticed before being hastily removed, if not for Google’s own search engine algorithm, which plucked the paper from its obscure server and emailed it to Dowling and anyone else who had signed up for Google Scholar alerts related to quantum computing.
The random number generator experiment may seem like an arbitrary benchmark for quantum supremacy without much practical application. But Aaronson has recently proposed that such a random quantum circuit could become the basis of a certified randomness protocol that could prove very useful for certain cryptocurrencies and cryptographic protocols. Beyond this very specific application, he suggests that future quantum computing experiments could aim to perform a useful quantum simulation of complex systems such as those found in condensed matter physics.
What’s Next for Quantum Computing?
Google’s apparent achievement doesn’t rule out the possibility of another research group developing a better classical computing algorithm that eventually solves the random number generator challenge faster than Google’s current quantum computing device. But even if that happens, quantum computing capabilities are expected to greatly outpace classical computing’s much more limited growth as time goes on.
“This horse race between classical computing and quantum computing is going to continue,” says Daniel Lidar, director of the Center for Quantum Information Science and Technology at the University of Southern California in Los Angeles. “Eventually though, because quantum computers that have sufficiently high fidelity components just scale better as far as we know—exponentially better for some problems—eventually it’s going to become impossible for classical computers to keep up.”
Google’s team has even coined a term to describe how quickly quantum computing could gain on classical computing: “Neven’s Law.” Unlike Moore’s Law that has predicted classical computing power will approximately double every two years—exponential growth—Neven’s Law describes how quantum computing seems to gain power far more rapidly through double exponential growth.
“If you’ve ever plotted a double exponential [on a graph], it looks like the line is zero and then you hit the corner of a box and you go straight up,” says Andrew Sornborger, a theoretical physicist who studies quantum computers at Los Alamos National Laboratory in New Mexico. “And so before and after, it’s not so much like an evolution, it’s more like an event—before you hit the corner and after you hit the corner.”
Quantum computing’s exponential growth advantage has the potential to transform certain areas of scientific research and real-world applications in the long run. For example, Sornborger anticipates being able to use future quantum computers to perform far more complex simulations that go well beyond anything that’s possible with today’s best supercomputers.
Wanted: Quantum Error Correction
Another long-term expectation is that a practical, general-purpose quantum computer could someday crack the standard digital codes used to safeguard computer security and the Internet. That possibility triggered premature alarm bells from conspiracy theorists and at least one U.S. presidential candidate when news first broke about Google’s quantum supremacy experiment via the Financial Times. (The growing swirl of online speculation eventually prompted Junye Huang, a Ph.D. candidate at the National University of Singapore, to share a copy of the leaked Google paper on his Google Drive account.)
In fact, the U.S. government is already taking steps to prepare for the future possibility of practical quantum computing breaking modern cryptography standards. The U.S. National Institute of Standards and Technology has been overseeing a process that challenges cryptography researchers to develop and test quantum-resistant algorithms that can continue to keep global communications secure.
The apparent quantum supremacy achievement marks just the first of many steps necessary to develop practical quantum computers. The fragility of qubits makes it challenging to maintain specific quantum states over longer periods of time when performing computational operations. That means it’s far from easy to cobble together large arrays involving the thousands or even millions of qubits that will likely be necessary for practical, general-purpose quantum computing.
Such huge qubit arrays will require error correction techniques that can detect and fix errors in the many individual qubits working together. A practical quantum computer will need to have full error correction and prove itself fault tolerant—immune to the errors in logical operations and qubit measurements—in order to truly unleash the power of quantum computing, Lidar says.
Many experts think the next big quantum computing milestone will be a successful demonstration of error correction in a quantum computing device that also achieves quantum supremacy. Google’s team is well-positioned to shoot for that goal given that its quantum computing architecture showcased in the latest experiment is built to accommodate "surface code” error correction. But it will almost certainly have plenty of company on the road ahead as many researchers look beyond quantum supremacy to the next milestones.
“You take one step at a time and you get to the top of Mount Everest,” Lidar says. “This is not the top of Mount Everest, but it’s certainly crossing a pretty big peak along the way.”
Jeremy Hsu has been working as a science and technology journalist in New York City since 2008. He has written on subjects as diverse as supercomputing and wearable electronics for IEEE Spectrum. When he’s not trying to wrap his head around the latest quantum computing news for Spectrum, he also contributes to a variety of publications such as Scientific American, Discover, Popular Science, and others. He is a graduate of New York University’s Science, Health & Environmental Reporting Program.