22 October 2019

Google Claims Quantum Supremacy - Not so Fast Says IBM, but are they Right?

What Google's Quantum Supremacy Claim Means for Quantum Computing

Leaked details about Google's quantum supremacy experiment stirred up a media frenzy about the next quantum computing milestone

Return to editingThe Limits of Quantum Computers

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.

Google’s new Bristlecone processor brings it one step closer to quantum supremacy

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.”

The Complexity of Quantum Sampling QIP 2018 Michael Bremner

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.

Quantum Computing and Quantum Supremacy

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.”

D Wave Webinar: A Machine of a Different Kind, Quantum Computing, 2019

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.”


“This is not the top of Mount Everest, but it’s certainly crossing a pretty big peak along the way.”
—Daniel Lidar, University of Southern California
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.

Introduction to Quantum Computing

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.

The Integration Algorithm A quantum computer could integrate a function in less computational time then a classical computer... The integral of a one dimensional.

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 moment when quantum computing can seriously threaten to compromise the security of digital communications remains many years, if not decades, in the future.
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.

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.”

This could be the dawn of a new era in computing. Google has claimed that its quantum computer performed a calculation that would be practically impossible for even the best supercomputer – in other words, it has attained quantum supremacy.

If true, it is big news. Quantum computers have the potential to change the way we design new materials, work out logistics, build artificial intelligence and break encryption. That is why firms like Google, Intel and IBM – along with plenty of start-ups – have been racing to reach this crucial milestone.

The development at Google is, however, shrouded in intrigue. A paper containing details of the work was posted to a NASA server last week, before being quickly removed. Several media outlets reported on the rumours, but Google hasn’t commented on them.

Read more: Revealed: Google’s plan for quantum computer supremacy
A copy of the paper seen by New Scientist contains details of a quantum processor called Sycamore that contains 54 superconducting quantum bits, or qubits. It claims that Sycamore has achieved quantum supremacy. The paper identifies only one author: John Martinis at the University of California, Santa Barbara, who is known to have partnered with Google to build the hardware for a quantum computer.

“This dramatic speedup relative to all known classical algorithms provides an experimental realization of quantum supremacy on a computational task and heralds the advent of a much-anticipated computing paradigm,” the paper says.

Google appears to have partnered with NASA to help test its quantum computer. In 2018, the two organisations made an agreement to do this, so the news isn’t entirely unexpected.

Making an impossible universe with IBM's quantum processor

The paper describes how Google’s quantum processor tackled a random sampling problem – that is, checking that a set of numbers has a truly random distribution. This is very difficult for a traditional computer when there are a lot of numbers involved.

But Sycamore does things differently. Although one of its qubits didn’t work, the remaining 53 were quantum entangled with one another and used to generate a set of binary digits and check their distribution was truly random. The paper calculates the task would have taken Summit, the world’s best supercomputer, 10,000 years – but Sycamore did it in 3 minutes and 20 seconds.

This benchmarking task isn’t particularly useful beyond producing truly random numbers – it was a proof of concept. But in the future, the quantum chip may be useful in the fields of machine learning, materials science and chemistry, says the paper. For example, when trying to model a chemical reaction or visualise the ways a new molecule may connect to others, quantum computers can handle the vast amount of variables to create an accurate simulation.

“Google’s recent update on the achievement of quantum supremacy is a notable mile marker as we continue to advance the potential of quantum computing,” said Jim Clarke at Intel Labs in a statement.

CQT11: The challenge of developing post-classical applications with noisy quantum computers

Yet we are still at “mile one of this marathon”, Clarke said. This demonstration is a proof of concept, but it isn’t free of errors within the processor. Better and bigger processors will continue to be built and used to do more useful calculations.

Read more: Google’s quantum computing plans threatened by IBM curveball
At the same time, classical computing isn’t giving up the fight. Over the past few years, as quantum computing took steps towards supremacy, classical computing moved the goal posts as researchers showed it was able to simulate ever more complex systems. It is likely that this back-and-forth will continue.

“We expect that lower simulation costs than reported here will eventually be achieved, but we also expect they will be consistently outpaced by hardware improvements on larger quantum processors,” says the Google paper.

A month ago, news broke that Google had reportedly achieved “quantum supremacy”: it had gotten a quantum computer to run a calculation that would take a classical computer an unfeasibly long time. While the calculation itself—essentially, a very specific technique for outputting random numbers—is about as useful as the Wright brothers’ 12-second first flight, it would be a milestone of similar significance, marking the dawn of an entirely new era of computing.

But in a blog post published today, IBM disputes Google’s claim. The task that Google says might take the world’s fastest classical supercomputer 10,000 years can actually, says IBM, be done in just days.

As John Preskill, the CalTech physicist who coined the term “quantum supremacy,” wrote in an article for Quanta magazine, Google specifically chose a very narrow task that a quantum computer would be good at and a classical computer is bad at. “This quantum computation has very little structure, which makes it harder for the classical computer to keep up, but also means that the answer is not very informative,” he wrote.

Google’s research paper hasn’t been published, but a draft was leaked online last month. In it, researchers say they got a machine with 53 quantum bits, or qubits, to do the calculation in 200 seconds. They also estimated that it would take the world’s most powerful supercomputer, the Summit machine at Oak Ridge National Laboratory, 10,000 years to repeat it with equal “fidelity,” or the same level of uncertainty as the inherently uncertain quantum system.

The problem is that such simulations aren’t just a matter of porting the code from a quantum computer to a classical one. They grow exponentially harder the more qubits you’re trying to simulate. For that reason, there are a lot of different techniques for optimizing the code to arrive at a good enough equivalent.

And that’s where Google and IBM differ. The IBM researchers propose a method that they say would take just two and a half days on a classical machine “with far greater fidelity,” and that “with additional refinements” this could come down even further.

Quantum Computing and Quantum Supremacy at Google

The key difference? Hard drives. Simulating a quantum computer in a classical one requires storing vast amounts of data in memory during the process to represent the condition of the quantum computer at any given moment. The less memory you have available, the more you have to slice up the task into stages, and the longer it takes. Google’s method, IBM says, relied heavily on storing that data in RAM, while IBM’s “uses both RAM and hard drive space.” It also proposes using a slew of other classical optimization techniques, in both hardware and software, to speed up the computation. To be fair, IBM hasn't tested it in practice, so it's hard to know if it would work as proposed. (Google declined to comment.)

So what’s at stake? Either a whole lot or not much, depending on how you look at it. As Preskill points out, the problem Google reportedly solved is of almost no practical consequence, and even as quantum computers get bigger, it will be a long time before they can solve any but the narrowest classes of problems. Ones that can crack modern codes will likely take decades to develop, at a minimum.

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Moreover, even if IBM is right that Google hasn’t achieved it this time, the quantum supremacy threshold is surely not far off. The fact that simulations get exponentially harder as you add qubits means it may only take a slightly larger quantum machine to get to the point of being truly unbeatable at something.

Still, as Preskill notes, even limited quantum supremacy is “a pivotal step in the quest for practical quantum computers.” Whoever ultimately achieves it will, like the Wright brothers, get to claim a place in history.

Every major tech company is looking at quantum computers as the next big breakthrough in computing. Teams at Google,  Microsoft, Intel, IBM and various startups and academic labs are racing to become the first to achieve quantum supremacy — that is, the point where a quantum computer can run certain algorithms faster than a classical computer ever could.

Quantum Computing Germany Meetup v1.0

Today, Google said that it believes that Bristlecone, its latest quantum processor, can put it on a path to reach quantum supremacy in the future. The purpose of Bristlecone, Google says, it to provide its researchers with a testbed “for research into system error rates and scalability of our qubit technology, as well as applications in quantum simulation, optimization, and machine learning.”

One of the major issues that all quantum computers have to contend with is error rates. Quantum computers typically run at extremely low temperatures (we’re talking millikelvins here) and are shielded from the environment because today’s quantum bits are still highly unstable and any noise can lead to errors.

Because of this, the qubits in modern quantum processors (the quantum computing versions of traditional bits) aren’t really single qubits but often a combination of numerous bits to help account for potential errors. Another limited factor right now is that most of these systems can only preserve their state for under 100 microseconds.

The systems that Google previously demonstrated showed an error rate of one percent for readout, 0.1 percent for single-qubit and 0.6 percent for two-qubit gates.

Quantum computing and the entanglement frontier

Every Bristlecone chip features 72 qubits. The general assumption in the industry is that it will take 49 qubits to achieve quantum supremacy, but Google also cautions that a quantum computer isn’t just about qubits. “Operating a device such as Bristlecone at low system error requires harmony between a full stack of technology ranging from software and control electronics to the processor itself,” the team writes today. “Getting this right requires careful systems engineering over several iterations.”

Google’s announcement today will put some new pressure on other teams that are also working on building functional quantum computers. What’s interesting about the current state of the industry is that everybody is taking different approaches.

Microsoft is currently a bit behind in that its team hasn’t actually produced a qubit yet, but once it does, its approach — which is very different from Google’s — could quickly lead to a 49 qubit machine. Microsoft is also working on a programming language for quantum computing. IBM currently has a 50-qubit machine in its labs and lets developers play with a cloud-based simulation of a quantum computer.

Technical quarrels between quantum computing experts rarely escape the field’s rarified community. Late Monday, though, IBM’s quantum team picked a highly public fight with Google.

In a technical paper and blog post, IBM took aim at potentially history-making scientific results accidentally leaked from a collaboration between Google and NASA last month. That draft paper claimed Google had reached a milestone dubbed “quantum supremacy”—a kind of drag race in which a quantum computer proves able to do something a conventional computer can’t.

Programming a quantum computer with Cirq (QuantumCasts)

Monday, Big Blue’s quantum PhDs said Google’s claim of quantum supremacy was flawed. IBM said Google had essentially rigged the race by not tapping the full power of modern supercomputers. “This threshold has not been met,” IBM’s blog post says. Google declined to comment.

It will take time for the quantum research community to dig through IBM’s claim and any responses from Google. For now, Jonathan Dowling, a professor at Louisiana State University, says IBM appears to have some merit. “Google picked a problem they thought to be really hard on a classical machine, but IBM now has demonstrated that the problem is not as hard as Google thought it was,” he says.

Whoever is proved right in the end, claims of quantum supremacy are largely academic for now. The problem crunched to show supremacy doesn’t need to have immediate practical applications. It's a milestone suggestive of the field’s long-term dream: That quantum computers will unlock new power and profits by enabling progress in tricky areas such as battery chemistry or health care. IBM has promoted its own quantum research program differently, highlighting partnerships with quantum-curious companies playing with its prototype hardware, such as JP Morgan, which this summer claimed to have figured out how to run financial risk calculations on IBM quantum hardware.

Quantum Computing 2019 Update

The IBM-Google quantretemps illustrates the paradoxical state of quantum computing. There has been a burst of progress in recent years, leading companies such as IBM, Google, Intel, and Microsoft to build large research teams. Google has claimed for years to be close to demonstrating quantum supremacy, a useful talking point as it competed with rivals to hire top experts and line up putative customers. Yet while quantum computers appear closer than ever, they remain far from practical use, and just how far isn’t easily determined.

The draft Google paper that appeared online last month described posing a statistical math problem to both the company’s prototype quantum processor, Sycamore, and the world’s fastest supercomputer, Summit, at Oak Ridge National Lab. The paper used the results to estimate that a top supercomputer would need approximately 10,000 years to match what Sycamore did in 200 seconds.

Classical simulation algorithms for quantum computational supremacy experiments

IBM, which developed Summit, says the supercomputer could have done that work in 2 ½ days, not millennia—and potentially even faster, given more time to finesse its implementation. That would still be slower than the time posted by Google’s Sycamore quantum chip, but the concept of quantum supremacy as originally conceived by Caltech professor John Preskill required the quantum challenger to do something that a classical computer could not do at all.

This is not the first time that Google’s rivals have questioned its quantum supremacy plans. In 2017, after the company said it was closing in on the milestone, IBM researchers published results that appeared to move the goalposts. Early in 2018, Google unveiled a new quantum chip called Bristlecone said to be ready to demonstrate supremacy. Soon, researchers from Chinese ecommerce company Alibaba, which has its own quantum computing program, released analysis claiming that the device could not do what Google said.

Google is expected to publish a peer-reviewed version of its leaked supremacy paper, based on the newer Sycamore chip, bringing its claim onto the scientific record. IBM’s paper released Monday is not yet peer reviewed either, but the company says it will be.

Did Google Just Achieve 'Quantum Supremacy'?

Jay Gambetta, one of IBM’s top quantum researchers and a coauthor on the paper, says he expects it to influence whether Google’s claims ultimately gain acceptance among technologists. Despite the provocative way IBM chose to air its technical concerns, he claims the company’s motivation is primarily to head off unhelpful expectations around the term “quantum supremacy,” not to antagonize Google. “Quantum computing is important and is going to change how computing is done,” Gambetta says. “Let’s focus on the road map without creating hype.”


Other physicists working on quantum computing agree that supremacy is not a top priority—but say IBM’s tussle with Google isn’t either.

“I don't much like these claims of quantum supremacy. What might be quantum supreme today could just be classical inferior tomorrow,” says Dowling of Louisiana State. “I am much more interested in what the machine can do for me on any particular problem.”

Chris Monroe, a University of Maryland professor and cofounder of quantum computing startup IonQ agrees. His company is more interested in demonstrating practical uses for early quantum hardware than academic disputes between two tech giants, he says. “We’re not going to lose much sleep over this debate,” he says.

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