Physicists entangle two moving helium atoms for the first time to test ‘spooky’ quantum theory

Scientists have observed quantum entanglement in the physical movement of atoms for the first time, bringing into sharper reality what Albert Einstein once described as “spooky motion at a distance.”

In the new study, published in the journal Nature Communications, researchers demonstrated that pairs of ultracold helium atoms can be quantum mechanically coupled through their momentum, a measure of the speed and direction of a particle’s movement taking into account its mass.

Quantum entanglement is one of the strangest features of quantum mechanics. If two particles are entangled, measurements of one will immediately affect the other. Scientists have previously demonstrated this with photons (fluxes of light) and the internal spin states of atoms, but not with the motion of particles with mass. This is important because atoms have mass, and mass responds to gravity. Photon is not like that. Atoms with entangled momentum could one day enable quantum sensors precise enough to detect ripples in space and time called gravitational waves and map the Earth’s interior.

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First, the researchers chose helium as the atom because it can maintain a long-lived excited state for about two hours, which is “essentially infinite” for experiments that last only 20 to 30 seconds, Sean Hodgman, an experimental physicist at the Australian National University and lead author of the study, told Live Science. This internal energy means that each atom hits the detector with enough force to be recorded individually. This will allow the team to reconstruct the full three-dimensional momentum of the cloud at single-atom resolution.

To create pairs of atoms with entangled momentum, the team started with a cloud of helium cooled to near absolute zero. Normally, atoms move around independently. But when it cools down enough, it slows down to almost a stop. Their quantum identities blur into a single collective object called a Bose-Einstein condensate.

They then used tuned laser pulses to split the condensate into three groups. One kicked upward, one downward, and one remained stationary. When a moving cloud passed through a stationary cloud, pairs of atoms collided and were scattered in opposite directions, forming spherical shells of correlated pairs. Physicists call this a “scatter halo.” At low enough densities, only one pair is scattered per experimental shot. “You either have a pair at one position, or you have a pair at another position,” Hodgman said. “Your entangled state is a combination of both.”

To prove that the entanglement is real, the research team used a device called a Rarity-Tapster interferometer. This method was first demonstrated using photons in 1990 and has now been extended to matter waves for the first time.

“The atoms scatter apart. Then they reflect back onto themselves and interfere together,” Hodgman explained. “Interference only occurs when an atom actually has both states superimposed.” The correlation the researchers measured cannot be explained by classical theory.

To arrive at the final results, the team collected data continuously for nearly a month and spent anywhere from a month to a year just setting up the experiment.

“This has been something of a long-term goal for our lab for probably 20 years or so,” Hodgman said. “We’re really excited to finally be able to demonstrate that.”

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The surreal triumph of quantum mechanics

Hodgman added that while the results were exciting, they primarily served to test “textbook” physical theories. Quantum mechanics predicts exactly this kind of behavior, but that doesn’t make it disorienting.

“Our brains aren’t equipped to process that,” Hodgman added. “The atoms appear smeared on a small scale, rather than as concrete clumps or little spheres. And that seems really, really weird.”

The team is already working on a more powerful version of the test. But the experiment that Hodgman describes as the most important next step involves colliding two isotopes of helium, helium-3 and helium-4, which are fundamentally different types of particles, creating pairs that are simultaneously entangled in both momentum and mass.

“From a quantum gravity perspective, how can we write a gravitational description of such a state?” Hodgman said. “It is completely unexplainable within the framework of general relativity. This kind of state would be a big challenge to explain with quantum gravity theory.”

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