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One atomic wave was held while another fell, directly measuring the quantum phase of free fall
Researchers split an ultracold rubidium atom wave into two paths. Magnetic fields held one path near an atom chip while the other was pushed upward and allowed to fall freely. Recombining them produced interference that directly measured the free-fall quantum phase, agreeing with the prediction obtained from Einstein’s equivalence principle.
The experiment did not unify general relativity and quantum mechanics, nor show that gravity itself is quantized. It is a precise low-energy, near-Earth test of whether quantum-wave evolution remains consistent with the equivalence principle.
What is the equivalence principle?
Locally, a freely falling observer should not feel gravity. The experiment compared a falling matter wave with one fixed in the laboratory.
How did the interferometer work?
Microwave pulses created a superposition; chip wires supported one component magnetically while the other followed a ballistic path, then the paths recombined.
What was new?
The team reports the first direct measurement of the predicted phase between freely falling and stationary wave packets, including its cubic dependence on fall time.
Why is this not quantum gravity?
The atom was quantum, but Earth’s gravitational field was treated classically. No superposed or entangling gravitational field was detected.
What comes next?
Heavier objects and longer superpositions, including planned nanodiamond tests, could probe where standard quantum mechanics might fail.
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