The Holy Grail of Quantum Computing Is Finally Here. Or Is It?

The world’s biggest computing companies and a raft of well-funded startups all agree: The future of computing is manipulating data with quantum mechanics. Over the past decade, governments, private companies, and venture capitalists have collectively invested billions of dollars into quantum computing, which aims to solve problems using a new type of logic enabled by harnessing quantum properties such as superposition and entanglement rather than ordinary 1’s and 0’s. Yet despite some prototypes capable of elementary operations, the hardware isn’t reliable enough to be practically useful.

Researchers from Google and Colorado-based startup Quantinuum independently announced results this year that advanced a long-sought idea tipped to solve quantum hardware’s flakiness. Both teams demonstrated a mechanism needed for a component called a topological qubit, which should offer a way to hold onto and manipulate information encoded into quantum states more robustly than current hardware designs. But did the two companies actually make the long-sought component? It depends who you ask.

Physicists developed the design of the topological qubit to reduce computational errors, enabling more complex algorithms and opening the door to the technology’s projected moneymaking applications, from drug discovery to financial modeling to more efficient AI. “This could well be a transistor moment for the quantum computing industry,” said Quantinuum founder Ilyas Khan in the company’s announcement. “We have used a quantum computer as the machine tool for building topological qubits.”

Google performed a similar demonstration to Quantinuum’s, using the company’s own quantum computer, but they refuse to label it in the same way. “We did not realize a topological qubit,” write Google researchers Trond Andersen and Yuri Lensky in an email to WIRED. To add to the multiplicity of interpretations of the experiments, one of Google’s collaborators holds a different view.

The disagreement over what exactly is happening inside some of the leading prototype quantum computers points to the hurdles faced by the nascent industry. Researchers have developed elegant designs and exciting potential applications for the budding machines, but they struggle to execute them in practice.

Physicist Sergey Frolov of the University of Pittsburgh, who was not involved in the work, has despaired of the hype and says it’s clear to him that neither Quantinuum nor Google has created a topological qubit. “It does not take quantum computing to the next level,” he says. While Google and Quantinuum demonstrated that their hardware can exhibit several hallmark features of a topological qubit, the components are too fragile to fulfill their intended role in quantum computing—to hold onto and manipulate information robustly. “To me, a topological qubit has protection,” Frolov says. “This doesn’t.”

Bold Claims

Google’s project to explore the underpinning of one of the holy grails of quantum computing emerged from a collaboration with a team at Cornell University led by physicist Eun-Ah Kim. They published the results in the scientific journal Nature in May.

Topological qubits store and work with digital information in quirky minuscule objects known as non-Abelian anyons, which emerge from the collective behavior of electrons or other particles confined to a flat surface. Unlike most particles that physicists and engineers work with, non-Abelian anyons are not defined by the material they’re made of but by how they behave owing to their geometric properties. The designation “anyon” is similar to “wave,” in that a wave is defined by its behavior and can consist of water, air, a metal guitar string, or a variety of materials. Researchers have proposed making non-Abelian anyons out of clusters of electrons, ions, neutral atoms, and superconducting circuits.

Most crucially, non-Abelian anyons retain a sort of “memory” of their past movement that can be used to represent binary data. At its simplest, a topological qubit is a system that encodes data into the properties of pairs of non-Abelian anyons by physically swapping them around with each other in space. These changes are robust to outside disturbances such as tiny vibrations or fluctuations in temperature that can overwhelm other qubits—resilience that is rooted in math from the field of topology, the study of spatial relationships and geometry that hold true even when shapes are distorted. This makes non-Abelian anyons desirable components for holding and manipulating information in quantum computers.

Kim’s team made non-Abelian anyons using 25 superconducting circuits that make up one of Google’s quantum computers. They showed that after moving the non-Abelian anyons around, they did retain a memory of their past motion. Kim says that leaves no room for dispute, as that “memory” is the device’s signature design feature. “We made a topological qubit,” she says.

Andersen and Lensky of Google disagree. They do not think the experiment demonstrates a topological qubit, because the object cannot reliably manipulate information to achieve practical quantum computing. “It is repeatedly stated explicitly in the manuscript that error correction must be included to achieve topological protection and that this would need to be done in future work,” they write to WIRED.

When WIRED spoke with Tony Uttley, the president and COO of Quantinuum, after the company’s own announcement in May, he was steadfast. “We created a topological qubit,” he said. (Uttley said last month that he was leaving the company.) The company’s experiments made non-Abelian anyons out of 27 ions of the metal ytterbium, suspended in electromagnetic fields. The team manipulated the ions to form non-Abelian anyons in a racetrack-shaped trap, and similar to the Google experiment, they demonstrated that the anyons could “remember” how they had moved. Quantinuum published its results in a preprint study on arXiv without peer review two days before Nature published Kim’s paper.

Room for Improvement

Ultimately, no one agrees whether the two demonstrations have created topological qubits because they haven’t agreed on what a topological qubit is—even if there is widespread agreement that such a thing is highly desirable. Consequently, Google and Quantinuum can perform similar experiments with similar results but end up with two very different stories to tell.

Regardless, Frolov at the University of Pittsburgh says that neither demonstration appears to have brought the field closer to the true technological purpose of a topological qubit. While Google and Quantinuum appear to have created and manipulated non-Abelian anyons, the underlying systems and materials used were too fragile for practical use.

David Pekker, another physicist at Pittsburgh, who previously used an IBM quantum computer to simulate the manipulation of non-Abelian anyons, says that the Google and Quantinuum projects don’t showcase any quantum advantage in computational power. The experiments don’t shift the field of quantum computing from where it has been for a while: Working on systems that are too small-scale to yet compete with existing computers. “My iPhone can simulate 27 qubits with higher fidelity than the Google machine can do with actual qubits,” Pekker says.

Still, technological breakthroughs sometimes grow from incremental progress. Delivering a practical topological qubit will require all kinds of studies—large and small—of non-Abelian anyons and the math underpinning their quirky behavior. Along the way, the quantum computing industry’s interest is helping further some fundamental questions in physics.

Nobel Laureate and theoretical physicist Frank Wilczek first proposed the existence of anyons, of which non-Abelian anyons are a specific type, in 1982, outside of the context of technology. Anyons, if they existed, constituted a fundamentally new category of matter, separate from previously recognized particles such as electrons and photons. Wilczek’s idea expanded the types of objects nature would allow, and for years physicists tried to conclusively show that non-Abelian anyons existed. While physicists previously found evidence pointing to their existence, Quantinuum’s and Google’s work are the first ever to demonstrate their signature feature, their “memory” of their movement.

In fact, Kim says she wanted to do this experiment to study anyons themselves, not because she was striving to build a better quantum computer. “My personal motivation was pure science, outside of any hype or any computing motivations,” she says.

As a theoretical physicist, Kim has thought deeply about quantum particles such as anyons to generate testable hypotheses about the strange ways they could behave in certain materials. Her work is motivated by a curiosity about how matter works. But for a while, it remained too difficult to manipulate materials in the laboratory with the level of control she needed. “I had kind of given up because the gap between the real world and where theorists comfortably live seemed too far,” she says.

Google’s quantum computer gave Kim a means to create anyons and test her hypotheses. The nascent quantum computing industry may have to wait a while longer until it gets practical topological qubits to start building with, but projects like Kim’s and Quantinuum’s are helping to deepen physicists’ understanding of non-Abelian anyons.

Source : Wired