Physicists measure quantum phase shift predicted in 1927
Physicists have measured for the first time a quantum phase shift that arises when an object falls in Earth’s gravitational field. The result of an experiment with ultracold rubidium atoms confirms that Einstein’s equivalence principle holds at the quantum level, Live Science reports.
The study was published on September 2 in the journal Science Advances. The equivalence principle, which underlies general relativity, states that locally it is impossible to distinguish the effects of gravity from acceleration.
Two trajectories of an atom
A team led by physicist Ron Folman of Ben-Gurion University of the Negev in Israel cooled about 20,000 rubidium atoms into a Bose-Einstein condensate state. Using radio and microwave pulses, the scientists placed the atoms into a superposition of two magnetic states.
One part of the atom was in free fall under Earth’s gravity. The other remained sensitive to a magnetic field, whose strength was adjusted to exactly compensate for gravity and keep it stationary relative to Earth. At the highest point of the trajectory, the two parts of the atom were separated by about 7.5 micrometers.
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Growth of the phase shift
After recombining the two parts of the atom, the researchers recorded an interference signal that showed the phase difference between them. Theory predicted that this shift would grow in proportion to the cube of the free-fall duration: if the falling time is doubled, the effect should increase eightfold.
The scientists increased the free-fall duration to about 2.4 milliseconds. Over 633 experimental runs during 5.3 hours, they recorded 13 complete phase oscillations. The measured growth matched the theoretical model to within about 2.5%.
Co-author Vlatko Vedral, a physicist at the University of Oxford, noted that the same result can be obtained through two approaches: by viewing gravity as a force acting on the quantum wave, or by moving to a free-falling reference frame and applying the equivalence principle. At the same time, the authors noted that the experiment does not rule out all theories in which this principle may be violated, as some of them also predict the same phase shift.
