March 24, 2026 -- In a new quantum physics experiment, ANU researchers have shown that matter can experience entanglement – an effect Einstein dismissed as ‘spooky action at a distance’.

The experiment, using helium atoms, is a major advance over similar entanglement experiments which used light – photons. Because atoms have mass, unlike photons, they offer future opportunities to study quantum effects and gravitational forces in the same experiment – two realms whose respective theories are famously incompatible.

Linking the two theories could lead to the elusive Unified Theory of Everything, which Einstein spent the last 30 years of his life searching for, unsuccessfully.

The experiment involved what is known as a Bell Inequality Test, in which the quantum properties of two particles are entangled. For the first time, this experiment entangled momentum of atoms, said lead author Yogesh Sridhar.

“The experiment pushes the limits of where quantum physics has been proven to apply,” said Mr Sridhar, a PhD student in the Department of Quantum Science and Technology.

“They show nonlocality in the external motion of atoms, rather than internal degrees of freedom such as spin.”

“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.”

The research is published in Nature Communications and is a collaboration between scientists at ANU, the University of Queensland and The University of Oklahoma in the United States.

The helium atoms used in the experiment are much more complicated entities than the photons used in previous momentum-based Bell Inequality tests. While photons are fundamental particles, helium atoms contain two protons, two neutrons and two electrons. The fact that this composite particle itself behaves as a matter wave is the ultimate test of spookiness, said lead researcher, Dr Sean Hodgman, from the Department of Quantum Science.

“Bell tests prove that entanglement is actually how the world works. 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 Hodgman said.

The experiment is carried out with three clouds of cold helium atoms, suspended in a trap of magnetic fields. The fields are turned off, allowing the atoms to begin falling under the force of gravity, and nudged towards each other with laser light. The light is pulsed in a way that forms a standing wave that acts as a grating, partially reflecting the atoms.  

The clouds pass through each other, and in the process, atoms collide, changing their momentum. Because the density is very low, on average only one pair collides. With the three initial states, there are a number of options for the final momentum of the colliding pairs.

The existence of the different options creates quantum entanglement: although different pairs follow different trajectories, ending up in different places, they are linked. Interactions with one option will affect other pair options, at a distance, in the spooky way that made Einstein uncomfortable.  

As the atoms fall, they encounter a series of grating laser pulses, creating a few different paths that the atoms can travel along, with equal probability.

The multiple path options of these falling pairs form an interferometer, in this configuration known as a Rarity-Tapster Interferometer. This setup allows quantum correlations to be measured. This is done by allowing the atoms to finally fall onto detectors: where they land depends on their momentum, what path they took and whether they were reflected.

The measurements clearly showed entanglement: the atoms within pairs were correlated in their momentum in a manner that shows the atom pair was split between multiple momentum states – the criterion for a Bell Inequality test.

The result confirms theories of quantum mechanics from a century ago, which hypothesised that matter can be in multiple locations at once and interfere with itself, even at large distances.

However, it is only in the recent decades that the technology to control and measure individual atoms has been developed, which prompted the researchers to attempt a Bell’s Inequality test for helium atoms.

The first design that they came up with was inspired by work in the group of 2022 Nobel Prize winner Alain Aspect.  Although it was theoretically sound when proposed by the theorists in the collaboration, it was difficult to implement and optimise experimentally. However, with some modifications it was made workable.

Demonstrating quantum entanglement with atoms – that is, with matter that feels gravity – opens the way to exploring the basis of a theory that could encompass the effects of both quantum mechanics and gravity, which is described by Einstein’s general theory of relativity.

“Imagine atoms moving through different paths in space, they can experience different gravitational effects,” Dr 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.

“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.”