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Wednesday, May 22, 2024

Two Physicists Bet Over a Quantum Computing Moon Shot

In February, two physicists made a bet on Twitter. Jonathan Dowling, a professor at Louisiana State University, and John Preskill of Caltech wagered a pizza and a beer over whether 10 years from now, someone will have finally invented a machine of longtime physics fantasy: the so-called topological quantum computer.

Preskill bet yes; Dowling bet no. “Preskill immediately liked the idea of the bet,” says Dowling, who initiated it. “He and I have been going at it, back and forth, on this topic for some time.”

To document the agreement, Dowling typed up the terms in PowerPoint on a clip art parchment backdrop. The specific terms of the bet are laid out in a blurry image inside a tweet on the two physicists’ feeds. (“I blame the pixelation on Preskill,” says Dowling. “He resaved it as a PDF.”) They will settle the bet on March 1, 2030, at midnight, Coordinated Universal Time.

To most, the subject of their bet is fairly esoteric. But among experts, the building of a topological quantum computer has been a decades-long moon shot, first championed by academics and later taken up by Microsoft, where researchers continue to pursue its development today. “It’s so beautiful and so elegant,” says Preskill, of topological quantum computing.

Indeed, “beautiful” and “elegant” are perhaps the words most commonly used by physicists to describe topological quantum computing. First proposed in 1997 by Russian-American physicist Alexei Kitaev, a topological quantum computer represents information in clusters of electrons, known as non-Abelian anyons, inside a material. Theory predicts that these clusters retain a sort of memory of their movement in the material, and the computer could encode information in how they are swapped around. For example, in a pair of anyons, a 0 might be represented as an anyon swapping positions with the one to its right, and a 1 would be swapping the right-side anyon over the left.

Physicists liken swapping two anyons to braiding two strands of hair. The bit of information is represented in which strand is on top in the braid, not in the physical properties of the hair itself. Information encoded this way is also much harder to alter, compared to conventional quantum computing. The quantum bits, or qubits, should commit far fewer errors relative to qubits based on the properties of physical objects, such as the superconducting circuits that make up Google’s and IBM’s noisy quantum machines. When these quantum computers execute complicated algorithms, for example, a circuit can accidentally change the property of its neighbor, producing an error, which researchers don’t quite know how to correct. Topological quantum computers would be resistant to this type of error.

Topological quantum computing exploits the field of geometry known as topology, hence its name. Topologists study properties of objects that stay the same despite deformation. For example, imagine shaping a piece of clay into the shape of a doughnut. You should be able to then smoothly morph that doughnut into the shape of a coffee cup without tearing or re-attaching any clay. Thus, a coffee cup and a doughnut are what’s known as topologically equivalent.

In the same way, a topological qubit will preserve its contained information as long as it remains in a topologically equivalent state, which means you can deform that qubit “as much as a doughnut is different from a coffee cup, and it still works,” says Dowling. Proponents say that such a machine would not suffer the computation errors that plague existing quantum machines—if only physicists could figure out how to build it.

Preskill learned of topological quantum computing in 1997, during Kitaev’s first visit to the US from Moscow, and he immediately fell in love with the idea. Previously, researchers thought the only way to avoid quantum computing errors was to implement an additional software algorithm that corrected the errors—algorithms that researchers are still working to develop. Kitaev, now Preskill’s colleague at Caltech, presented a design that protects the computer from errors by virtue of the hardware itself, without the need for extra error-correcting code. His machine would use qubits that could be stretched and deformed, so to speak, while retaining their information.

“Kitaev had a very specific and beautiful idea for correcting errors at the hardware level,” says Preskill. “I thought, this is going to be the answer in the long run.”

Dowling, however, thinks that supporters of the idea need a reality check. “He is very enthralled with the beautiful mathematics,” says Dowling, speaking of Preskill. Dowling compares topological quantum computing to string theory, a once-popular approach to unifying all the laws of physics that has since fallen out of favor because its ideas are impossible to test experimentally.

“I agree that I’m drawn to it because it’s mathematically elegant,” agrees Preskill. “What’s wrong with that?”

The physicists have known each other for more than two decades, and they have both worked on quantum computing research for even longer. Today, Preskill has become, in a way, the fatherly public face of quantum computing in the US. In his patient drawl, he often explains quantum concepts to the popular press and nonspecialists at venture capitalist firms. In this de facto role, Preskill has coined catchphrases to make concepts in the math-heavy field easier to grasp. Other quantum computing researchers have adopted his terms, such as “NISQ” (pronounced “nisk”), which stands for noisy intermediate-scale quantum and describes the state of existing quantum computers. He also coined the controversial term “quantum supremacy” to broadly describe a task in which a quantum computer surpasses a conventional computer. Google’s quantum computing team used the term in 2019 to describe their landmark experiment, but many in the field find it offensive because it evokes the phrasing of “white supremacy.”

“I was a bit taken aback that people found it offensive,” says Preskill. “I understand now that people do react that way.” He now opts for the phrase “quantum computational supremacy” to clarify that he is not talking about the supremacy of any group of people.

Meanwhile, Dowling has titled himself a community “bullshit detector.” He’s quantum computing’s trollish uncle, perhaps. He endorses or condemns claims about physics experiments on Twitter with “yes,” “no,” “hype,” “not hype,” and the occasional “not even wrong”—certainly the most searing of physicist insults. “Jon is … how should I say it? An outspoken person,” says Preskill. “He is often a critic of what other people say about quantum science.”

Both of them have made scientific bets in the past. Preskill’s most famous bet was with the late physicist Stephen Hawking over whether information about objects swallowed by a black hole could later be retrieved. (Preskill won the bet in 2004, when Hawking conceded that yes, information can escape from black holes.)

Dowling’s bets sometimes contain ulterior motives. “Back in 1999, I bet a guy from the NSA that we would have a quantum computer in 10 years that they could use for something,” says Dowling, who worked at the Army Research Office at the time. He lost, as 2009 came and went—but he says the bet served its purpose. He thinks the bet helped increase research funding for quantum computing. “When this guy’s managers at the NSA asked him, ‘When will quantum computers be ready?’ he would say, ‘Some people say never, but this one crazy guy working at the Army says in 10 years,’” he recalls. Dowling speculates that the specificity of his bet spurred funders to feel competitive, and according to him, NSA’s quantum computing research funds tripled within a year of the bet. “The bet did what I wanted it to do,” he says.

Scientists have long used wagers to spur problem-solving. As early as 1600, astronomer Johannes Kepler bet his mentor Tycho Brahe that he could calculate Mars’ orbit around the sun in five days. (Kepler lost the bet, but he figured it out five years later.) In more recent years, the Long Now Foundation, a nonprofit whose mission is to spur long-term thinking, has established a Long Bet project, a website where people can keep track of wagers, which include ones predicting the world population and the future of slaughterhouses in the UK. The competition helps to focus people’s attention on difficult problems.

Dowling’s quippy musings aside, he has a few solid points to support his skepticism of topological quantum computing. First of all, researchers have not conclusively demonstrated they can actually produce non-Abelian anyons in a material, to make electrons form that distinct pattern. This means they still cannot build the fundamental component of a topological quantum computer. In addition, some of the anyons’ purported advantages only exist when they are kept at impractically cold temperatures near absolute zero. In this respect, they are no better than quantum computers like Google’s built from superconducting circuits.

The current bet arose from one of Dowling’s Twitter diatribes, in which he accused Microsoft of overhyping its progress. Microsoft, the only big company investing in topological quantum computing, has been working to create non-Abelian anyons in materials since 2005, when the company established Station Q, a research center in Santa Barbara, California, dedicated to this endeavor. In 2012, 2016, and 2018, Microsoft-affiliated researchers presented promising but inconclusive results in the journals Science and Nature, writing that they’d puppeteered electrons in tiny wires stuck to superconductors into anyon-like patterns.

A Microsoft representative declined an interview request for this article. But in November, Microsoft’s general manager for quantum hardware Chetan Nayak told WIRED, “We’re very excited about the progress that we’ve been making.”

To Dowling, this progress is unpromising. “Microsoft put all of their resources into this, and they have nothing to show for it,” he says. “OK, that’s not bad—sometimes you bet on the wrong horse, and you get on with your life. But every year I hear someone from Microsoft give a talk, saying that they’re just a few days away from an important announcement, that they’ve demonstrated these non-Abelian anyons. And no announcement ever comes. At what point do they say this doesn’t work?”

Preskill agrees that the work is taking longer than expected, but he doesn’t share Dowling’s cynicism. “I am confident we’ll have topological quantum computers at some time in the future, but the time scale is still very uncertain,” he says.

If Dowling wins, he says he’d request a New York style pizza and a German wheat beer. However, he admits that he doesn’t really care about the outcome of the bet. “I would be really happy if I lost,” says Dowling. “That would mean somebody made a topological quantum computer.” The point of the wager is “to focus the community in a fun way on a particular unsolved problem,” he says.

Preskill hasn’t considered pizza toppings or beer styles yet. For him, bets are also a little game he plays with the public. “It gets the public interested in scientific issues,” he says. If he and Dowling can get you to think about topological quantum computing today for a couple seconds more than you usually do, they’ve already won.

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