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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Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.