# Glitz & Qubits

When the first email came, Alexei Kitaev ignored it. The subject heading said something about a physics award, but he thought it was just spam. “Then I received another email,” says the Caltech physicist. “So I actually took a look and understood that it was real.” Real it was. Kitaev had won the first ever Breakthrough Prize in Fundamental Physics, established in 2012 by Russian billionaire entrepreneur Yuri Milner. And this new prize came with $3 million—three times what winners of the Nobel Prize get. Unlike the Nobels, the money isn’t shared among the winners, of which there were eight others. “I couldn’t believe that each person received $3 million,” Kitaev says.

Milner meant the award to come with a significant chunk of change; his goal is not only to recognize scientists doing fundamental research, but also to raise their profiles among the general public to equal the likes of actors, sports stars, and other celebrities. “We have a disbalance in the world today that the best minds are not appreciated enough,” Milner said at the 2013 prize ceremony.

Indeed, the big payout got plenty of media attention. It was probably one of the few times, if not the only time, that the topic of topological quantum computing—Kitaev’s expertise—graced the pages of the Los Angeles Times.

**A Quantum Future**

Kitaev won the prize for laying some of the theoretical groundwork for quantum computers—machines that researchers say can far exceed the performance of conventional computers. By exploiting the complexities intrinsic to quantum mechanics, quantum computers could hypothetically do calculations that would take regular computers the entire age of the universe to accomplish. It’s not hyperbole to say that, some day, quantum computers could change everything.

By comparison, conventional computers are relatively simple. They process information by turning a bunch of electronic switches on and off, with those states—units of information called bits—represented by a 1 (on) or 0 (off). Quantum computers are different; instead of simple on/off switches, they rely on quantum bits called qubits, which follow the weird rules of quantum mechanics.

Quantum theory has been a triumph: it forms the backbone of modern physics, explaining everything from the LED display on your TV to the MRI technique that doctors use to examine your insides. But the concepts are notoriously nonintuitive.

For example, one of the theory’s principles is superposition, in which an object can occupy two states simultaneously. In other words, an electron can be spinning clockwise and counterclockwise at the same time. While a bit can be only on or off, a qubit can be both, a combination of 0 and 1. Then there’s entanglement, in which two particles can be so intimately correlated that one gives you information about the other—even if they’re separated by the length of the universe. Entangled qubits become inextricably correlated with one another.

Needless to say, computing with qubits is complicated. But it’s this mind-bending complexity that endows a quantum computer with its unsurpassed processing power—in principle, at least. Today’s quantum computers are rudimentary, only capable of relatively simple tasks (e.g. factoring the number 21). No one’s been able to engineer one that’s particularly useful—that is, one that achieves what a regular computer can’t. In a conventional computer, bits manifest themselves as tiny electronic switches called transistors, which are embedded in small silicon chips. Qubits, on the other hand, are really hard to make.

Researchers have tried to accomplish this by using different kinds of quantum systems, including charged atoms, whose clockwise or counterclockwise spins provide the two states of a qubit. But regardless of the design, quantum systems are extremely fragile, susceptible to stray particles that can bump into a qubit and ruin whatever calculation it’s trying to make. These errors quickly accumulate, rendering the computer useless.

The first line of defense is to develop algorithms that weed out the errors. But these corrections are so difficult to do that the computer ends up spending much of its computational muscle fixing errors, rather than doing its intended task. Then Kitaev came up with a better solution—one worth $3 million.

He devised a way to build error-resistance into the computer’s hardware itself, instead of relying on software to correct problems after-the-fact. According to Kitaev, you could construct qubits with exotic particles called anyons, which are thought to exist in certain quantum systems. Two anyons can share a single quantum state, and maintain that state even if separated. Their identities are so intertwined that if a stray particle wanted to disturb the qubit, it would have to perturb both anyons—which is harder to do, especially if the two anyons are separated. So by keeping the anyons apart, you can make an error-resistant qubit.

Kitaev proposed his idea in 1997, attracting enough attention to go on to win a MacArthur Fellowship in 2008. Since then, he says, physicists have realized that making qubits from actual anyons may be too difficult. Instead, researchers such as Caltech’s Jason Alicea and Gil Refael are pursuing ways to apply Kitaev’s ideas using another particle-like quantum object called a Majorana mode.

Such a qubit would be made of a thin, superconducting wire that traps a pair of Majorana modes, one on each end. To calculate something, the computer would change the voltage inside the wires, moving those Majorana modes around and thus manipulating the qubits’ quantum states.

This general strategy, which Kitaev helped pioneer, is called topological quantum computing. While others are using it to design a bona fide quantum computer, he’s focusing on the theory behind it, trying to gain a deeper mathematical understanding of how these topological systems work.

Topology is like a fancier version of geometry; it’s a mathematical study of spaces and the properties of those spaces, which can occupy any number of dimensions. Topologically, a doughnut is the same as a coffee mug, since they have the same basic property of having a hole (the mug’s handle forms its hole).

In the case of quantum computing, topology comes in a bit more abstractly. With Kitaev’s proposal, for example, information processing would happen when anyons (or Majorana modes) move around. Their motion through time can be depicted in what’s called a space-time diagram, in which an x-axis and a y-axis represent space, while time lies on a third axis. A pair of stationary anyons would be represented by two parallel lines pointing up. But if one anyon moves around the other in the x-y plane, their motion would be depicted as two lines wrapped around each other like a braid. By studying the topology of these braids, Kitaev can explore the mathematical properties of these systems and how they could function as a quantum computer.

While a real, useful quantum computer could still be decades away, Kitaev’s work has been fundamental, carving out a new potential path toward such a machine. Which, of course, is why in 2013 he found himself onstage with eight other preeminent physicists—and actor Morgan Freeman.

The aspirations behind the Breakthrough Prizes, to imbue glitz and glamour to basic science, were clear as the evening proceeded like a Hollywood-style, red-carpet event. “You can think of it as being like the Oscars,” said Freeman, who hosted the ceremony. “Only this time, you’re in the presence of some of the greatest minds on the planet.”

**Stringing It All Together**

The next year, Kevin Spacey hosted the affair, which also saw Glenn Close and Conan O’Brien—among other Hollywood celebrities—in attendance. And Caltech had two more representatives: biologist Alexander Varshavsky, who won the 2014 Breakthrough Prize in Life Sciences *(read more at eands. caltech.edu/plus)** *and another theoretical physicist, John Schwarz, who won the 2014 Breakthrough Prize in Fundamental Physics.

Schwarz and his corecipient, Michael Green of the University of Cambridge, were recognized for perhaps the most fundamental kind of physics: their efforts to develop a unified theory that describes all the basic forces and particles of nature— a theory of everything.

Physicists from Albert Einstein to Stephen Hawking have searched in vain for a grand unified theory. And while such a theory remains elusive, physicists like Schwarz have made tremendous progress over the decades. Indeed, the best—and only— candidate for a unified theory today is string theory, an idea that Schwarz pioneered in the 1970s.

According to string theory, the fundamental building blocks of reality consist of vibrating stringlike objects. These strings have different properties, such as tension, that determine how they vibrate. And those modes of vibration, like notes on a plucked guitar string, are the elementary particles of the universe.

But when physicists were first developing the theory in the 1960s, they had no inkling it could be a theory of everything. Instead, they were trying to use it to explain the aptly named strong force, which holds atomic nuclei together. After physicists like Caltech Nobel laureate David Politzer developed the theory of quantum chromodynamics to describe the strong force, researchers tossed string theory aside. Except for Schwarz.

He thought the mathematical beauty of string theory must hint at some underlying,fundamental truth. It took more tinkering—with the late Joel Scherk, who was a senior research fellow at Caltech at the time—to find out what that truth was. In 1974, Schwarz and Scherk realized that string theory predicted the existence of a particle that resembles the graviton, a hypothetical particle thought to be responsible for gravity. That prediction was a breakthrough because in the efforts to unify the four fundamental forces—the strong force, the weak force, the electromagnetic force, and gravity—incorporating gravity was the biggest hurdle.

Compared to the other forces, gravity is a problem child. Physicists were able to describe and even unify the other forces using quantum mechanics. But gravity, which Einstein described as the warping of the spacetime fabric of the cosmos, wouldn’t play nice with quantum mechanics. Any attempts at a quantum theory of gravity gave nonsensical results incompatible with reality.

So when Schwarz and Scherk showed that string theory predicted the graviton—and thus gravity—they also realized it could be the long-sought unified theory. But the excitement was short-lived. They soon ran into mathematical inconsistencies in the theory that would stymie the field for a decade.

Schwarz and his longtime collaborator Green (Scherk died in 1979) became essentially the only ones pushing forward with string theory. “It was viewed quite skeptically by most of the theoretical physics community,” Schwarz says. “But some people seemed to realize it was a worthwhile gamble.”

Luckily, one of those people was Caltech’s Murray Gell-Mann, who had helped bring Schwarz to Caltech in 1972 as a research associate. With Gell-Mann’s support, Schwarz kept working on his theory. Finally, in 1984, he and Green found a way to cancel out the previously problematic mathematical inconsistencies, removing what had been a major roadblock.

“John is a visionary,” says Hirosi Ooguri, the Fred Kavli Professor of Theoretical Physics and Mathematics at Caltech. “He pushed this for 10 years, and then finally found a solution that convinced the rest of the world that this is the right direction.”

Suddenly, string theory became one of the most exciting areas of physics, and physicists like Ooguri wanted to be part of it. Today, the field continues to attract interest from younger generations of scientists. “There’s more optimism than ever,” Schwarz says. But despite decades of progress, researchers still have a way to go. One major reason for skepticism, for instance, is the lack of experimental evidence.

But many physicists are hanging their hopes on the Large Hadron Collider, located under the border of France and Switzerland. As the most powerful particle accelerator in the world continues to ramp up its current round of collisions, which will run through 2018, Schwarz hopes it will discover evidence for supersymmetry, a theory that, among other things, posits that every particle has a supersymmetric partner (an electron, for example, has a partner called the selectron) to help explain why particles have mass.

Supersymmetry is a feature of nature that’s necessary for string theory. “I would say the probability is on the order of 50 percent or so that it will show up,” Schwarz says. “If they do find supersymmetry at the LHC, this would be absolutely revolutionary in terms of impact on fundamental particle physics. It would pretty much set the agenda for the experimental side of particle physics for the next half century.”

The cachet of an award like the Breakthrough Prize in Fundamental Physics also helps set the agenda, by bringing much-needed attention to this basic research. “It’s important that the United States continues to be a leader in high-energy physics,” Schwarz says. “Public recognition of a field of science is good. It helps make decision-makers in the government and so on more cognizant of the work and more predisposed to support it.”

As for the Hollywood effect, Schwarz thinks it might take more than a couple of awards ceremonies to turn theoretical physicists into household names. At the 2014 award ceremony, Schwarz says, he and his wife, Patricia, were “both struck by the fact that the Hollywood types showed no interest in mingling with scientists.” And the media coverage also seemed to focus on the movie stars rather than the award winners.

But for Kitaev, the biggest impact of awards like the Breakthrough Prize in Fundamental Physics is on his family. “They don’t really understand what I’m working on,” he says. But thanks to these awards, they at least realize his research is a pretty big deal. “It helps me do more work because they have more respect for it,” he says. “My wife is really proud of me.”

*Written by Marcus Woo*

*Alexei Kitaev** is the Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics. His work is supported by the Simons Foundation, the National Science Foundation, and Gordon and Betty Moore Foundation.**John Schwarz** is the Harold Brown Professor of Theoretical Physics, Emeritus. He remains an active participant in Caltech’s Walter Burke Institute for Theoretical Physics.*