The logic gates in a future topological quantum computer would be based on the spacetime braids of two-dimensional quasiparticles called anyons as they move around. Mathematicians have done considerable work on braids, which are depicted abstractly here.The logic gates in a future topological quantum computer would be based on the spacetime braids of two-dimensional quasiparticles called anyons as they move around. Mathematicians have done considerable work on braids, which are depicted abstractly here.Illustration: Ester Dalvit

It’s not yet possible to build a general-purpose quantum computer with enough quantum bits (qubits) to do anything useful, but many researchers hope eventually to achieve that goal. They’re mostly using one of two strategies to store and manipulate qubits: superconducting loops or trapped ions. Both of these approaches are prone to error, though, making it necessary to build in much redundancy so that the needed error correction can be done. It might ultimately require thousands of physical qubits working together to make up a single error-corrected logical qubit. And that could make it impossible to construct a general-purpose quantum computer.

That’s why some researchers (in particular, some working at Microsoft) have long considered a third strategy: building a “topological” quantum computer, which, if it were ever to come to pass, would have qubits that were much less prone to error. Constructing a topological quantum computer would, however, be much harder than what’s being explored now with superconducting loops and trapped ions. Indeed, it’s very hard even to describe what a topological supercomputer uses for qubits.

If you try looking up the answer on Wikipedia, you’ll be faced with a wall of jargon that might make you suspect that Alan Sokal is playing another joke: “A topological quantum computer is a theoretical quantum computer that employs two-dimensional quasiparticles called anyons, whose world lines pass around one another to form braids in a three-dimensional spacetime….” As much as it resembles a line from a bad science fiction film, this sentence, in fact, describes a serious proposition for a way to build a quantum computer. At some point in the future when I understand it more, I promise to travel back through spacetime and revise this paragraph to make the concept abundantly clear.

In the meantime, let me share news of a development that could one day make it easier to build a topological quantum computer. For that, you need topological qubits, and to construct topological qubits, you need a very special physical substrate. While it’s possible to construct such a substrate using thin layers of different materials, a better approach might be to use a substance that in bulk acts as what is known as a topological superconductor.

“Those are very rare,” says Nick Butch, who is a condensed-matter physicist at the National Institute of Standards and Technology and also an adjunct associate professor of physics at the University of Maryland. He and his colleagues study new and unique types of superconductors, and they believe that they have discovered one that may indeed prove to be a topological superconductor, uranium ditelluride. Their work is published online today in the journal Science.

Butch and his colleagues decided to investigate uranium ditelluride because they suspected that it might be magnetic at very low temperatures, ones approaching absolute zero. It turns out that it isn’t magnetic at such low temperatures, but to their surprise, the researchers found that it becomes a superconductor. “This was quite an accident,” says Butch.

The surprises continued as they studied the new superconductor further. You see, superconductors stop superconducting when subjected to a magnetic field that exceeds some critical value. For a typical superconductor, the critical magnetic field intensity might be a few teslas. But uranium ditelluride remains a superconductor in magnetic fields as high as 20 T. Indeed, the researchers were unable to create a magnetic field of sufficient intensity in their laboratory to quash its superconductivity.

This experimental observation implies that the superconductivity of uranium telluride comes about in a special way. Let me explain. Superconductors conduct electricity without resistance because the current-carrying electrons pair up to form what are known as Cooper pairs (after Leon Cooper, who with John Bardeen and John Robert Schrieffer developed the first microscopic theory of superconductivity in 1957). In most superconductors, the two electrons in a Cooper pair will have opposing “spins”—a quantum mechanical property that describes the particle’s inherent magnetism. In a few odd superconductors, though, the electrons in these Cooper pairs will have spins that point in the same direction, which for complicated reasons is called a spin triplet state.

Butch explains that “a large subset of those [superconductors] that have spin triplets are topological superconductors.” That’s why he and his colleagues are so excited about their recent chance discovery: If they have indeed stumbled on a material that is a topological superconductor, it may one day serve as the basis for building topological qubits.

Sure, that’s a lot of ifs. And Butch readily admits that it is not yet completely clear that the Cooper pairs in superconducting uranium ditelluride are in a triplet state or, if so, that the material is indeed a topological superconductor. “We’ve built a circumstantial case” for that, he says.

What if that case falls apart with further measurements? Or worse, what if that case holds up, as seems likely, but the notion of topological quantum computing fizzles for other reasons, leaving no obvious application for the new material? Would superconducting uranium ditelluride still be interesting? Butch answers in a way you might expect from a basic-science researcher: “It’s interesting because it exists.”

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The Spectacular Collapse of CryptoKitties, the First Big Blockchain Game

A cautionary tale of NFTs, Ethereum, and cryptocurrency security

8 min read
Mountains and cresting waves made of cartoon cats and large green coins.
Frank Stockton

On 4 September 2018, someone known only as Rabono bought an angry cartoon cat named Dragon for 600 ether—an amount of Ethereum cryptocurrency worth about US $170,000 at the time, or $745,000 at the cryptocurrency’s value in July 2022.

It was by far the highest transaction yet for a nonfungible token (NFT), the then-new concept of a unique digital asset. And it was a headline-grabbing opportunity for CryptoKitties, the world’s first blockchain gaming hit. But the sky-high transaction obscured a more difficult truth: CryptoKitties was dying, and it had been for some time.

The launch of CryptoKitties drove up the value of Ether and the number of transactions on its blockchain. Even as the game's transaction volume plummeted, the number of Ethereum transactions continued to rise, possibly because of the arrival of multiple copycat NFT games.

That perhaps unrealistic wish becomes impossible once the downward spiral begins. Players, feeling no other attachment to the game than growing an investment, quickly flee and don’t return.

Whereas some blockchain games have seemingly ignored the perils of CryptoKitties’ quick growth and long decline, others have learned from the strain it placed on the Ethereum network. Most blockchain games now use a sidechain, a blockchain that exists independently but connects to another, more prominent “parent” blockchain. The chains are connected by a bridge that facilitates the transfer of tokens between each chain. This prevents a rise in fees on the primary blockchain, as all game activity occurs on the sidechain.

Yet even this new strategy comes with problems, because sidechains are proving to be less secure than the parent blockchain. An attack on Ronin, the sidechain used by Axie Infinity, let the hackers get away with the equivalent of $600 million. Polygon, another sidechain often used by blockchain games, had to patch an exploit that put $850 million at risk and pay a bug bounty of $2 million to the hacker who spotted the issue. Players who own NFTs on a sidechain are now warily eyeing its security.

Remember Dragon

The cryptocurrency wallet that owns the near million dollar kitten Dragon now holds barely 30 dollars’ worth of ether and hasn’t traded in NFTs for years. Wallets are anonymous, so it’s possible the person behind the wallet moved on to another. Still, it’s hard not to see the wallet’s inactivity as a sign that, for Rabono, the fun didn’t last.

Whether blockchain games and NFTs shoot to the moon or fall to zero, Bladon remains proud of what CryptoKitties accomplished and hopeful it nudged the blockchain industry in a more approachable direction.

“Before CryptoKitties, if you were to say ‘blockchain,’ everyone would have assumed you’re talking about cryptocurrency,” says Bladon. “What I’m proudest of is that it was something genuinely novel. There was real technical innovation, and seemingly, a real culture impact.”

This article was corrected on 11 August 2022 to give the correct date of Bryce Bladon's departure from Dapper Labs.

This article appears in the September 2022 print issue as “The Spectacular Collapse of CryptoKitties.”

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