A quantum interferometer measures a falling atom's phase to within 2.5 per cent of Einstein's prediction
Physicists led by Ron Folman at Ben-Gurion University split about 20,000 rubidium atoms into a superposition, held one half still against gravity and let the other fall freely, then recombined the two to read the quantum phase Charles Galton Darwin and Earle Kennard predicted in 1927. The phase matched Einstein's equivalence principle to about 2.5 per cent. Folman calls the result a step toward reconciling gravity and quantum theory; the group's next target is larger objects such as nanodiamonds.
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20,000 atoms split, fall, and reveal a century-old phase
The interferometer split roughly 20,000 ultracold rubidium atoms into a superposition about 113 micrometres below an atom chip patterned with gold wires. One half was held in place by magnetic fields tuned against gravity while the other was pushed upward and allowed to fall freely, the two halves standing about 7.5 micrometres apart before being recombined across 633 cycles spanning 5.3 hours. The phase this motion builds up, written down in 1927 by Charles Galton Darwin and Earle Kennard, had never been read off an instrument; the measured value grew with the cube of the free-fall time and agreed with the prediction to about 2.5 per cent.[1]
Folman: a step toward gravity and quantum theory, not a union of them
The result extends Einstein's equivalence principle, which holds that an observer in free fall locally feels weightless, to a quantum object able to occupy two paths at once for the first time. Ron Folman of Ben-Gurion University, the study's lead author, describes the agreement as a step toward addressing how gravity and quantum theory might fit into one description of nature, but stresses the experiment does not unify general relativity with quantum mechanics or show that gravity itself is quantum. What it shows is narrower: within the tested conditions, the equivalence principle held for a quantum system.[2]
A practical ceiling, and a Penrose question the atoms are too light to reach
Recombining the two halves of each atom proved the hardest step: interference contrast started at 80 per cent on short runs and faded to 20 per cent on the longest, setting a practical ceiling on the method, and the authors caution the result does not rule out every equivalence-principle-violating theory that predicts the same phase. The experiment does not test Roger Penrose's proposal that quantum superpositions collapse once objects grow massive enough, since the rubidium atoms are far lighter and the run times far shorter than that would require; the same Ben-Gurion group is pursuing a related experiment with larger objects such as nanodiamonds.[1], [2]
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