First Measured Momentum Of Entangled Atoms Proves Theory That Particles Can Be In Two Places At Once

The capacity to entangle subatomic particles so that changes to one instantly affect the other – even over a distance – has now been demonstrated many times, despite it being one of the strangest features of modern physics. However, this has either been done with massless photons, or using internal features of atoms or electrons, such as their spin.

Despite the importance of these demonstrations of entanglement for science, they can’t address the question of how entanglement interacts with gravity. That changes when the feature of atomic behavior that has been entangled is momentum. This fact inspired a team led by Dr Sean Hodgman of the Australian National University (ANU) on a quest to explore the entanglement of helium atoms’ momentum.

“The experiment pushes the limits of where quantum physics has been proven to apply,” said ANU PhD student Yogesh Sridhar in an emailed 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 two separated atoms that are entangled, if you change one of them, it will instantly affect the other. 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!”

Dr Sean Hodgman

The authors supercooled two clouds of helium atoms to make Bose-Einstein Condensates (BECs) and pushed them towards each other. When atoms from each BEC met, they became entangled, although each cloud was so diffuse that that only happened for about one pair per run. Hodgson told IFLScience that two classical objects making such a collision would go either left/right or up/down, but quantum mechanics means they do both at once. Measuring the momentum of one atom collapses not only its own wavefunction, but also that of the one it was entangled with.

The team had the atoms fall into an interferometer, with their landing site determined by their momentum. The distribution of landing sites is only possible if, prior to measurement, the atoms were in more than one momentum state at once – effectively in two locations simultaneously – and their wavefunctions interfered with each other.

“For two separated atoms that are entangled, if you change one of them, it will instantly affect the other,” Hodgman said “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!”

Hodgman told IFLScience that helium atoms were chosen because they can be trapped in their first excited state. “This means they have high internal energy and release electrons we can measure, allowing us to measure the atoms with full three-dimensional resolution.”

Hodgson, Sridhar, and co-authors are so keen to explore the effects of gravity on entanglement because two of the most cherished theories in physics, quantum mechanics and general relativity, appear incompatible. Yet General Relativity passes every test we can throw at it when it comes to describing gravity, while quantum mechanics reigns supreme on the scale of the very small, despite the extent to which it defies common sense.

The search for a so-called “Theory of Everything” that would unite the two has motivated scientists since Einstein, but remains unfinished.

“Imagine atoms moving through different paths in space, they can experience different gravitational effects,” Hodgman explained. “However, quantum mechanics says atoms can take multiple paths simultaneously. How do you describe such a system in a general relativity framework? What does the space time curvature for such a system look like? No one really knows, because quantum and gravity don't match up nicely, although a lot of researchers are working on it.”

In theory, scaled up versions of the equipment the team used for this work could produce experimental answers to such questions.

The authors note, however, “An important aspect in any such future demonstration would be to ensure large space-like separation between the correlated atoms, required for closing the locality loophole.” In other words, the particles need to be distant enough to rule out any possibility the particles are influencing each other through some sub-lightspeed communication.

This would require the atoms to be separated by at least 30 centimeters (12 inches) and probably more. The existing equipment has a diameter of 8 centimeters (3.2 inches). By entangling atoms of different mass, for example helium-3 and helium-4 isotopes, the team suggest it would be possible to test important aspects of how quantum mechanics is affected by gravitational fields.

Hodgman told IFLScience that the team “would need a lot more funding to scale up,” and probably years of work reach the scale required to close the locality loophole.

The study is published open access in Nature Communications.