Quantum mechanics and relativity meet in new atom interferometer experiment

Physicists have successfully conducted an experiment using a novel atom interferometer to test the compatibility between quantum mechanics and Einstein's theory of general relativity. The experiment, led by Ron Folman at Ben-Gurion University of the Negev with international collaborators, utilized a new interferometer design to place a single atom into a superposition of two distinct paths simultaneously: one where it falls freely under gravity, and another where it remains stationary. This setup allows for the measurement of the phase difference accumulated by the falling atom, a key aspect for understanding how gravity affects quantum wave properties.

For decades, testing this theoretical intersection has been impossible due to the lack of suitable interferometers and the difficulty in cooling atoms to the near-absolute-zero temperatures required for their wave-like properties to manifest. The Quantum Galileo Interferometer (QGI) developed for this experiment uses microwave and magnetic pulses on about 20,000 rubidium atoms, cooled to a Bose-Einstein condensate. A microwave pulse creates a superposition of states, a magnetic pulse then acts as a 'cannon' to propel one wave packet upward, while the other remains stationary. Subsequent pulses manipulate the states so the falling packet experiences gravity without other forces, and the stationary one is held in place. A final pulse acts as a 'parachute' to slow the falling atom as it returns to the starting point.

The results showed that the phase accumulated by the falling atom was 2.5 percent off the theoretical prediction, a finding the team considers a crucial checkpoint at the boundary of these two fundamental theories. This experiment demonstrates that the equivalence principle, a cornerstone of relativity, can hold true even within a quantum system existing in superposition, a feat never before shown.