Quantum physicists exploit entanglement to improve the precision of optical atomic clocks

Imagine walking into a room where several different grandfather clocks hang on the walls, each ticking at a different pace. Quantum physicists at the University of Colorado Boulder and the National Institute of Standards and Technology (NIST) have essentially recreated this piece on the scale of atoms and electrons. The team’s progress could pave the way for new types 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 up of a few dozen strontium atoms trapped in a network. To improve the device’s performance, the team generated a type of ghostly interaction, known as quantum entanglement, between groups of these atoms, by crushing four different types of clocks in the same device. timing.

This is no 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 calls the “Holy Grail” for optical atomic clocks.

“What we can do is divide the same duration into smaller and smaller units,” said Kaufman, lead author of the new study and a member of JILA, a joint research institute between CU Boulder and NIST. “This acceleration could allow us to track time more precisely.”

The team’s progress 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 JILA graduate student, published their findings Oct. 9 in the journal Nature.

Atoms lassoed

The research represents another major advance for optical atomic clocks, which can do much more than tell time.

To make such a device, scientists typically start by trapping and cooling a cloud of atoms to freezing temperatures. They then destroy these atoms with a powerful laser. If the laser is perfectly tuned, the electrons orbiting these atoms will move from a lower energy level to a higher energy level and then back again. Think of it like the pendulum of a grandfather clock swinging back and forth: these clocks rotate over a trillion times per second.

They are extremely precise. JILA’s newest optical atomic clocks, for example, can detect the change in gravity if you lift them 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 a very high degree, both where those atoms are and also what states they are in.” find,” Kaufman said.

But they also have a major drawback: In quantum physics, objects as small as atoms never behave exactly as you would expect. These natural uncertainties set what appears to be an unbreakable limit to the precision a clock can achieve.

Entanglement could be a workaround, however.

Fluffy eye sockets

Kaufman explained that when two particles become entangled, information about one of them automatically reveals information about the other. In practice, the entangled atoms in a clock behave less like individuals than like a single atom, making their behavior easier to predict.

In the current study, the researchers generated this type of quantum bond by nudging their strontium atoms so that their electrons gravitated away from their nuclei, almost as if they were made of cotton candy.

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

These united pairs also work at a faster rate than single atoms.

The team experimented with creating clocks comprising a combination of individual atoms and entangled groups of two, four and eight atoms, in other words, four clocks running at four speeds in one.

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

“This means it takes us less time to achieve the same level of accuracy,” he said.

Exquisite control

He and his colleagues still have a lot of work to do. For starters, researchers can only run their clock effectively for about 3 milliseconds. Any longer than that, and the entanglement between the atoms begins to slip, making the atomic ticking chaotic.

But Kaufman sees great potential in this device. His team’s approach to atom entanglement could, for example, form the basis of what physicists call “multi-qubit gates”: the basic operations that perform calculations in quantum computers, or devices that could one day outperform traditional computers in certain tasks.

“The question is: can we create new types of clocks with tailored properties, made possible by the exquisite control we have in these systems?” » Kaufman said.