Physicists at the Australian National University (ANU) have observed quantum entanglement in the external motion of helium atoms, marking a significant advancement in the study of quantum phenomena. This experiment, which successfully utilized massive particles rather than light, provides new opportunities to investigate the intersection of quantum mechanics and gravitational forces, potentially contributing to a unified theory of physics.
Observation of Entanglement in Matter
Researchers at the ANU demonstrated quantum entanglement in matter using helium atoms. This observation marks an advancement from prior experiments that primarily utilized photons, which are light particles. Unlike photons, helium atoms possess mass and are subject to gravitational forces.
The phenomenon of entanglement involves particles existing in multiple states or locations simultaneously, a concept described as counter-intuitive by researchers.
Experimentally demonstrating this effect was characterized as highly challenging, with previous attempts by other groups not achieving this specific observation.
Experimental Methodology
The experiment involved a Bell Inequality Test, which was designed to entangle the momentum of atoms. Lead author Yogesh Sridhar stated that this work expands the application of quantum physics by demonstrating nonlocality in the external motion of atoms, rather than internal properties such as spin.
Dr. Sean Hodgman, a lead researcher, highlighted that the use of helium atoms, which are composite particles containing protons, neutrons, and electrons, represents a more rigorous test of quantum phenomena compared to fundamental photons.
Bell tests confirm entanglement, indicating that if one of two separated, entangled atoms is altered, the other is instantly affected.
The experimental setup involved suspending three clouds of cold helium atoms in magnetic fields. After the magnetic fields were disengaged, the atoms began to fall under gravity and were directed towards each other using laser light. This laser light created a standing wave that acted as a grating, partially reflecting the atoms.
As the clouds passed through each other, atoms collided, altering their momentum. Due to the low density of the clouds, typically only one pair of atoms collided, leading to various possible final momentum states for the colliding pairs. This interaction generated quantum entanglement, linking the pairs despite their subsequent divergent trajectories.
As the atoms continued to fall, they encountered a sequence of grating laser pulses, which created multiple possible paths for them to travel with equal probability. These multiple path options formed an interferometer, specifically a Rarity-Tapster Interferometer, enabling the measurement of quantum correlations. Detectors recorded where the atoms landed, providing data based on their momentum, path, and reflection status.
Findings and Implications
The measurements provided evidence of entanglement, showing that atoms within pairs were correlated in their momentum in a way that indicated the atom pair was split between multiple momentum states, satisfying the criteria for a Bell Inequality test.
These findings reinforce quantum mechanics theories established a century ago, which posited that matter can exist in multiple locations simultaneously and interact with itself, even across considerable distances. The development of advanced technology capable of controlling and measuring individual atoms made this experiment feasible.
Demonstrating quantum entanglement with atoms, which are subject to gravity, opens pathways for exploring a theoretical framework that could encompass both quantum mechanics and Albert Einstein's general theory of relativity.
Dr. Hodgman posed a key question for future research:
"How would space-time curvature be described for a system where atoms take multiple paths simultaneously, as predicted by quantum mechanics, given the current incompatibility between quantum theory and gravity?"
He noted that this research could enable the testing of gravitational effects within entangled systems.