'Really, really weird': Physicists entangle two moving atoms for the first time, validating 'spooky' quantum theory

For the first time, scientists have observed quantum entanglement in the way atoms physically move — bringing a phenomenon once described by Albert Einstein as "spooky action at a distance" into even sharper reality.

In the new study, published in the journal Nature Communications, researchers demonstrated that pairs of ultracold helium atoms can be quantum mechanically linked through their momentum — a measure of how fast and in which direction a particle moves, factoring in its mass.

Quantum entanglement is one of the strangest features of quantum mechanics. When two particles are entangled, a measurement of one instantly affects the other. Scientists had demonstrated this before in photons (packets of light) and in the internal spin states of atoms but never in the motion of particles with mass. This is important because atoms have mass, and mass responds to gravity; photons don't. Momentum-entangled atoms could one day power quantum sensors precise enough to detect space-time ripples called gravitational waves or to map Earth's interior.

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Catching entanglement in the act

First, the team chose helium as their atom, because it can be held in a long-lived excited state with a lifetime of around two hours — which is “essentially infinite” in experiments that only last 20 to 30 seconds, Sean Hodgman, an experimental physicist at the Australian National University and senior author of the study, told Live Science. That internal energy means each atom hits a detector with enough force to register individually. It allows the team to reconstruct the full three-dimensional momentum of the cloud with single-atom resolution.

To create momentum-entangled atom pairs, the team started with a cloud of helium cooled to near absolute zero. Normally, atoms zip around independently. But if you cool them enough, they slow to a near standstill. Their quantum identities blur together into a single collective object called a Bose-Einstein condensate.

Then, they used tuned laser pulses to split that condensate into three groups: one kicked upward, one kicked downward, and one left stationary. As the moving clouds passed through the stationary one, pairs of atoms collided and scattered in opposite directions, forming spherical shells of correlated pairs. Physicists call it "scattering halos." At low enough density, only a single pair scatters per experimental shot. "You either have a pair at one position, or a pair at another," Hodgman said. "Your entangled state is a superposition of both."

To prove the entanglement was real, the team used a device called a Rarity-Tapster interferometer. This method, first demonstrated with photons in 1990, now extended to matter waves for the first time.

"The atoms scatter apart; then you reflect them back onto themselves and interfere with them together," Hodgman explained. "Interference only occurs if the atom is truly in a superposition of both states." The correlations the team measured cannot be explained by any classical theory.

To get their final result, the team collected data continuously for nearly a month and spent a month to a year just setting up the experiment.

"This has kind of been a long-term goal for our lab for probably 20 years or so," Hodgman said. "To be able to finally demonstrate it is really exciting."

A surreal win for quantum mechanics

The result, while exciting, mainly served to validate “textbook” physics theories, Hodgman added. Quantum mechanics predicts this exact kind of behavior, but that doesn't make it any less disorienting.

"Our brains aren't really equipped to process it," Hodgman added. "Atoms appear as smeared out at small scales, not concrete blobs or little balls. And that just seems really, really weird."

The team is already working on a stronger version of the test. But the experiment Hodgman describes as the most consequential next step involves colliding two isotopes of helium ‪—‬ helium-3 and helium-4, which are fundamentally different kinds of particles — to create pairs entangled in both momentum and mass simultaneously.

"From a quantum gravity point of view, how do you even write down the gravitational description of that kind of state?" Hodgman said. "You can't really describe it in a general relativity framework at all. These sorts of states would provide a real challenge for quantum gravity theories to explain."

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