Observation of the quantum phase of free fall and the consistency with the equivalence principle

How do quantum mechanics and the general theory of relativity—specifically the principle of equivalence—interact at the level of a single particle? While the principle of equivalence is well-established for large bodies, its application to quantum wave packets has been a subject of theoretical debate for nearly a century. The authors sought to experimentally measure the predicted quantum phase of a free-falling object to determine if the equivalence principle holds in the quantum domain.

To test this, the authors developed a novel cold-atom interferometer called the Quantum Galileo Interferometer (QGI). Unlike traditional interferometers that might treat both paths symmetrically, the QGI creates a coherent superposition of two wave packets: one that remains stationary in the laboratory frame (held by magnetic gradients) and one that is in free fall. By precisely controlling the magnetic fields on an atom chip, the researchers could manipulate these wave packets to recombine them after a period of free fall, allowing them to observe the resulting interference pattern and extract the relative phase.

The experiment successfully observed the predicted cubic phase dependence on the free-fall time. The measured phase difference between the stationary and free-falling wave packets aligns with the theoretical predictions derived from the equivalence principle. This result demonstrates that, at the low energies and masses tested, the equivalence principle is consistent with quantum mechanics. The authors emphasize that this measurement provides a fundamental test of the interface between quantum theory and gravity, serving as a potential foundation for future experiments exploring the quantum-gravity boundary.

This study bridges a long-standing gap between theoretical predictions and experimental observation. By confirming that a free-falling quantum object accumulates the phase predicted by the equivalence principle, the researchers provide empirical support for the coexistence of quantum mechanics and general relativity in this regime. The QGI setup also offers a flexible platform for future research, including tests of more complex quantum equivalence principle formulations and searches for new physics, such as gravitationally induced wave-function collapse.