Bell inequality violation with momentum-entangled massive particles

For decades, physicists have been locked in a quiet debate about the fundamental nature of reality. On one side stands the idea of local realism, a common-sense view that objects have definite properties whether we look at them or not, and that nothing can influence something else faster than the speed of light. On the other side is quantum mechanics, the rulebook for the very small, which suggests that particles can be linked in ways that defy these limits. This link, known as entanglement, means that measuring one particle instantly reveals the state of its partner, no matter how far apart they are. In the 1960s, a physicist named John Bell devised a way to test which view was correct. He created a mathematical boundary, now called a Bell inequality, that any theory based on local realism must obey. If an experiment breaks this boundary, it proves that the universe does not follow the rules of local realism. Over the last fifty years, scientists have repeatedly broken this boundary using light particles, or photons, and the internal spin states of heavier particles like atoms. However, they have never done so using the actual movement of these heavy particles through space.

A team of researchers at the Australian National University and the University of Queensland has now closed this gap. They have performed the first test of Bell's theorem using the motion of massive atoms. Instead of looking at the internal spin of an atom, they looked at where the atoms were going and how fast they were moving. By creating pairs of helium atoms that were entangled in their momentum, the researchers showed that the atoms' movements were correlated in a way that is impossible to explain with local realism. They measured a value that exceeded the limit set by classical physics by three standard deviations, confirming that the strange, non-local connections of quantum mechanics apply to the physical motion of matter just as they do to light.

The experiment began with a cloud of helium atoms cooled to temperatures near absolute zero, forming a state of matter known as a Bose-Einstein condensate. In this state, the atoms behave like a single giant wave rather than individual particles. The researchers released this cloud from a magnetic trap and let it fall freely. They then used pulses of laser light to nudge the atoms, causing them to collide and scatter. These collisions were carefully controlled to produce pairs of atoms that flew off in opposite directions. Because of the laws of conservation, if one atom in a pair moved in a certain direction, its partner had to move in the exact opposite direction. This process created two spherical shells of atoms, known as scattering halos, where every atom on one side of the shell had a twin on the opposite side, moving in perfect opposition.

The challenge was to measure these moving atoms in a way that could test Bell's theorem. Previous attempts with massive particles relied on measuring internal properties like spin, which are easier to manipulate. Measuring the motion itself is much harder because the atoms are moving through space, and the tools used to measure them usually affect the entire group at once. To solve this, the team developed a new method using a dual-resonant laser lattice. This technique allowed them to address two specific groups of atoms within the scattering halos independently. They could apply a laser pulse to the top half of the shell and a different pulse to the bottom half, changing the phase, or timing, of the atoms' waves in each region without affecting the other. This independent control was the key missing piece that had prevented such a test until now.

Once the atoms were prepared, the researchers used a matter-wave interferometer, a device that splits and recombines the atomic waves, to measure the correlations between the pairs. They set up four different combinations of settings for the laser pulses and counted how often atoms from the top and bottom regions arrived at specific detectors together. If the atoms followed the rules of local realism, the correlations between their arrival times would stay below a certain limit. However, the data showed a clear violation of this limit. The researchers calculated a value of 2.52, which is significantly higher than the maximum value of 2 allowed by local realism. The uncertainty in their measurement was small enough that the result stands as a robust confirmation of quantum nonlocality in the motion of massive particles.

This achievement is more than just a technical milestone; it changes the scope of what we know about quantum mechanics. For the first time, we have proof that the spooky connections predicted by quantum theory govern the actual trajectory of matter through space, not just its internal states. The experiment demonstrates that the motion of a heavy atom is subject to the same fundamental quantum rules as a photon of light. The researchers used approximately 38,000 experimental shots to gather their data, ensuring that the result was not a fluke. They also verified that their findings were consistent across different sizes of detection areas, ruling out errors caused by how they grouped the data.

The implications of this work extend beyond the test itself. The ability to independently control the phases of moving atoms opens the door to new types of precision measurements. It provides a powerful platform for studying how quantum mechanics interacts with gravity, a field that remains one of the greatest unsolved mysteries in physics. By using massive particles that are sensitive to gravitational fields, scientists can now design experiments to see if gravity affects quantum entanglement in ways that light cannot reveal. The researchers also noted that this setup could be used to test entanglement between different isotopes of helium, offering a unique way to explore the relationship between mass, gravity, and quantum nonlocality. This work completes a long-standing objective in the field of atom optics, moving the study of quantum weirdness from the internal world of atoms to the external world of their motion.