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In a world first, scientists have experimentally verified the entanglement of the momentum of atoms, a feat previously only achieved with photons.
Scientists from Australia and the U.S. pulled off this immensely tricky measurement using ultracold helium atoms and an immensely complicated set-up known as a Rarity-Tapster Interferometer.
Because atoms have mass (unlike photons), this discovery opens up a door to studying gravitational effects in the quantum realm.
The universe—at least, as we know it—appears to be governed by two overarching principles. In the world of celestial mechanics, Albert Einstein’s General Theory of Relativity explains the relationship between mass and energy, and in the realm of the subatomic, quantum mechanics run the show. For nearly a century, however, there’s been a pretty big problem with this universal formulation—the two theories don’t really mesh.
One of the key sticking points is gravity—Einstein argues that it is an effect caused by the curvature of spacetime, while quantum physicists hypothesize that a (currently theoretical) force carrier known as a graviton mediates gravitational interaction. For nearly a century, scientists have tried to find evidence that might knit these two grand theories together, but so far, a happy marriage has remained impossible.
Now, a new study conducted by scientists at the Australian National University (ANU) in Canberra, the University of Queensland, and the University of Oklahoma has, for the first time, experimentally confirmed that the momentum of atoms can be entangled. While scientists have known about the entanglement of atoms (and the entanglement of photon momentum) for decades, this first confirmation of atom momentum entanglement is crucial to our understanding of the phenomenon—largely because atoms contain mass, and can thus provide a way for scientists to study quantum effects and gravitational forces in the same experiment. The results of the study were published in the journal Nature Communications.
“They show nonlocality in the external motion of atoms, rather than internal degrees of freedom such as spin,” Yogesh Sridhar, lead author of the study and a Ph.D. student from ANU, said in a press statement. “These results strengthen our confidence and understanding in quantum theory and also pave the way to testing quantum mechanical theories with even larger real-world objects.”
For the experiment, Sridhar, lead researcher Sean Hodgman (also from ANU), and the rest of the team used three clouds of cold helium atoms suspended in a magnetic trap. Once the magnets were turned off, the atoms fell under the influence of gravity and passed a series of grating laser pulses that created different paths along which the atoms could travel with equal probability. This set-up is what’s known as a Rarity-Tapster Interferometer (a specifically designed optical device for measuring non-locality, or entanglement) within what’s known as a Bell inequality-test framework (which tests for the non-local nature of particles, as opposed to Einstein’s local realism).
“For two separated atoms that are entangled, if you change one of them, it will instantly affect the other,” Hodgman said in a press statement. “It’s kind of crazy to think that this is how the world works, but we’ve shown that it’s the nature of reality.”
Of course, it’s been long hypothesized that matter can be at multiple places at once—at least, in a sense—and still interact with itself over large distances (Einstein once famously described this concept as “spooky action a distance”). But scientists have only been able to test that hypothesis in recent years because of advances in methods for controlling and measuring individual atoms.
“Imagine atoms moving through different paths in space, they can experience different gravitational effects. However, quantum mechanics says that atoms can take multiple paths simultaneously,” Hodgman said in a press statement. “So the fact that we can now show that these sort of systems are entangled means that we could then think about potentially looking at some gravitational effects that we could test with them.”
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