�鶹��Ժ

From left to right, Adam Kaufman, Nelson Darkwah Oppong, Alec Cao and Theo Lukin Yelin inspect an optical atomic clock at JILA on the CU Boulder campus. (Credit: Patrick Campbell/CU Boulder)

Imagine walking into a room where several different grandfather clocks hang on the walls, each ticking at a different pace. 

Quantum physicists at CU Boulder and the National Institute of Standards and Technology (NIST) have essentially recreated that room at the scale of atoms and electrons. The team’s advancement could pave the way for new kinds of optical atomic clocks, devices that track the passage of time by measuring the natural “ticking” of atoms.

The group’s new clock is made from a few dozen strontium atoms trapped in a lattice pattern. To improve the device’s performance, the team generated a type of ghostly interaction, known as quantum entanglement, between groups of those atoms—basically squishing four different kinds of clocks into the same time-keeping apparatus.

It’s not your ordinary pocket watch: The researchers showed that, at least under a narrow range of conditions, their clock could beat a benchmark for precision called the “standard quantum limit”—what physicist Adam Kaufman refers to as the “Holy Grail” for optical atomic clocks.

“What we’re able to do is divide the same length of time into smaller and smaller units,” said Kaufman, senior author of the new study and a fellow at between CU Boulder and NIST. “That acceleration could allow us to track time more precisely.”The team’s advancements could lead to new quantum technologies. They include sensors that can measure subtle changes in the environment, such as how Earth’s gravity shifts with elevation.

Kaufman and his colleagues, including first author Alec Cao, a graduate student at JILA, in the journal Nature.

Graduate students Theo Lukin Yelin and Alec Cao monitor an optical atomic clock at JILA. (Credit: Patrick Campbell/CU Boulder)

Lassoing atoms

The research represents another major advancement for optical atomic clocks, which can do a lot more than tell time.

To make such a device, scientists typically begin by trapping and chilling a cloud of atoms down to frigid temperatures. They then zap those atoms with a powerful laser. If the laser is tuned just right, electrons orbiting those atoms will jump from a lower energy level to a higher energy level, then back again. Think of it like the pendulum of a grandfather clock swinging back and forth—only these clocks tick more than a trillion times per second.

They’re extremely precise. The , for example, can detect the change in gravity if you lift them up by just a fraction of a millimeter.

“Optical clocks have become an important platform in many areas of quantum physics because they allow you to control individual atoms to such a high degree—both where those atoms are, and also what states they’re in,” Kaufman said. 

But they also have a big drawback: In quantum physics, things as small as atoms never behave exactly like you’d expect. These natural uncertainties set what seems to be an unbreakable limit on just how precise a clock can get. 

Entanglement, however, could provide a workaround.

Fluffy orbits

Kaufman explained that when two particles become entangled, information about one of them will automatically reveal information about the other. In practice, entangled atoms in a clock behave less like individuals and more like a single atom, which makes their behavior easier to predict.

In the current study, the researchers generated this kind of quantum link by nudging their strontium atoms so that their electrons orbited far away from their nuclei—almost as if they were made of cotton candy.

Lukin Yelin and Cao in the Kaufman lab at JILA. (Credit: Patrick Campbell/CU Boulder) 

“It’s like a fluffy orbit,” Kaufman said. “This fluffiness means that if you bring two atoms close enough, the electrons can feel each other nearby, resulting in a strong interaction between them.”

Those conjoined pairs also tick at a faster pace than atoms on their own.

The team experimented with creating clocks that included a combination of individual atoms and entangled groups of two, four and eight atoms—in other words, four clocks ticking at four rates in one.

They found that, at least under certain conditions, entangled atoms have a lot less uncertainty in their ticking than the atoms in a traditional optical atomic clock.

“That means that it takes us less time to get to the same level of precision,” he said.

Exquisite control

He and his colleagues still have a lot of work to do. For a start, the researchers can only run their clock effectively for about 3 milliseconds. Longer than that, and the entanglement between atoms starts to slip, causing the atomic ticking to become chaotic.

But Kaufman sees a lot of potential for the device. His team’s approach toward entangling atoms could, for example, form the basis for what physicists call “multi-qubit gates”—the basic operations that perform calculations in quantum computers, or devices that could one day outperform traditional computers at certain tasks. 

“The question is: Can we create new kinds of clocks with tailored properties, enabled by the exquisite control that we have in these systems?” Kaufman said.

Other CU Boulder and NIST co-authors on the study included NIST and JILA Fellow Jun Ye; JILA post-doctoral researchers Kyungtae Kim and Nelson Darkwah Oppong; and JILA graduate students William Eckner, Theo Lukin Yelin, Aaron Young and Lingfeng Yan. Guido Pupillo of the University of Strasbourg was also a co-author.