Quantum Gate 100x Faster Than Quantum Noise

Superfast operations may help neutral-atom-based machines outrun disruptions

4 min read
Illustration of two qubits in blue light with red light vertically

In this conceptual diagram of the world’s fastest two-qubit gate, two atoms, separated by a couple of micrometers, are captured by optical tweezers [red light] and manipulated by a superfast, 10-picosecond laser pulse [blue light].

Takafumi Tomita/Institute for Molecular Science

Quantum computers theoretically can solve problems no regular computer might ever hope to solve. However, the key ingredients of most quantum computers—quantum bits, or qubits, tied together by quantum entanglement—are highly vulnerable to disruption from their surroundings. Now scientists in Japan have successfully executed an operation with two qubits in just 6.5 nanoseconds—the fastest ever, which may essentially outrun the effects of any outside interference.

Classical computers switch transistors either on or off to symbolize data as ones or zeroes. In contrast, quantum computers use quantum bits or qubits, which because of the strange nature of quantum physics can exist in a state called superposition where they are both 1 and 0 at the same time. This essentially lets each qubit perform two calculations at once.

However, quantum computers are notoriously fragile to outside interference, such as electronic, ionic, or thermal fluctuations. This means present-day state-of-the-art quantum computers are highly prone to mistakes, typically suffering roughly one error every 1,000 operations. In contrast, many practical applications demand error rates lower by a billionfold or more.

“We can manipulate [neutral atom qubits] on completely new timescales, and it redefines what can be done with this platform.”
—Sylvain de Leseleuc, the Institute for Molecular Science, Okazaki, Japan

One way to deal with the effects of noise in quantum computers is to speed up the rate at which they perform elementary operations known as quantum gates—the quantum-computing version of the logic gates that conventional computers use to perform computations. The chance that a quantum gate will experience a mistake from noise grows over time, so the faster they operate, the lower the probability they will fail.

In the new study, researchers experimented with qubits composed of neutrally charged rubidium atoms. Neutral atoms may possess a number of benefits as qubits in comparison with other quantum computing platforms.

For instance, qubits based on atoms benefit from the way these particles are virtually all identical. In contrast, qubits based on devices, such as the superconducting circuits that Google and IBM uses in their quantum computers, must cope with the problems that result from the variations between these components that inevitably result during fabrication.

Another quantum-computing platform that has attracted growing interest uses electromagnetically trapped electrically charged ions. However, ions repel each other, making it difficult to stack them in a dense manner. By comparison, scientists can pack neutral atoms closer together.

In addition, the fact that neutral atoms lack electric charge means they do not interact easily with other atoms. This make them more immune to noise and means they can stay coherent, or in superposition, for a relatively long time. For example, in May, Berkeley, Calif.–based quantum-computing startup Atom Computing revealed they could keep neutral atom qubits coherent for roughly 40 seconds, the longest coherence time ever demonstrated on a commercial platform. Moreover, neutral atoms can get cooled with lasers instead of the bulky refrigeration needed with a number of other qubit platforms, such as superconducting circuits.

The scientists first trapped and cooled neutral atoms with arrays of laser beams. They next used these lasers to excite electrons to so-called Rydberg orbitals far from their atomic nuclei. The resulting “Rydberg atoms” can be hundreds to thousands of times as large as the atoms would be in their ground states.

In theory, the giant nature of Rydberg orbitals can lead Rydberg atoms to strongly experience interactions such as entanglement with each other, enabling rapid quantum gates, says study senior author Kenji Ohmori, a quantum physicist at the Institute for Molecular Science in Okazaki, Japan. However, previously no one had realized this possibility because of factors such as the stringent requirements for the positions of the atoms.

In the new study, the researchers used laser beams to control the distance between atoms with a precision of 30 nanometers. They also cooled the atoms to an ultralow temperature about 1/100,000 of a degree above absolute zero, to reduce any jittering from heat.

The researchers next used ultrashort laser pulses that lasted just 10 picoseconds—trillionths of a second—to excite a pair of these atoms to a Rydberg state at the same time. This let them execute a quantum gate entangling the qubits in just 6.5 ns, making it the fastest quantum gate to date. (The previous speed record for a quantum gate was 15 ns, achieved by Google in 2020 with superconducting circuits.)

“We can manipulate Rydberg atoms on completely new timescales, and it redefines what can be done with this platform,” says study coauthor Sylvain de Leseleuc, a quantum physicist at the Institute for Molecular Science in Okazaki, Japan.

Rydberg-atom quantum computers typically experience an error rate from noise of a few percent per microsecond, de Leseleuc says. This new two-qubit gate is hundreds of times as fast as this error rate, suggesting that quantum computers built using this strategy may ignore the effects of noise.

Although the researchers could space the Rydberg atoms anywhere from 1.5 to 5 micrometers apart, they ultimately chose a distance of roughly 2.4 µm. The interactions between Rydberg atoms becomes stronger the closer they are, de Leseleuc says. This means a shorter distance would lead to a faster gate that was less sensitive to external noise but more difficult to control, while a greater distance would lead to a slower gate more sensitive to external noise but less difficult to control, he explains.

Future work may aim for even faster, more reliable performance with a more stable laser whose energy fluctuates less than the commercial device used in these experiments, de Leseleuc says.

“We are opening a new playground with Rydberg atoms that we could call ‘ultrafast Rydberg physics’ as well as ‘ultrafast Rydberg quantum engineering,’ ” Ohmori says.

The scientists detailed their findings online 8 August in the journal Nature Photonics.

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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
Vertical
Mountains and cresting waves made of cartoon cats and large green coins.
Frank Stockton
Pink

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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