'Zeno effect' verified: Atoms won't move while you watch

22 October 2015

Graduate students Airlia Shaffer, Yogesh Patil and Harry Cheung work in the Ultracold Lab of Mukund Vengalattore, assistant professor of physics.

One of the oddest predictions of quantum theory - that a system can’t change while you’re watching it - has been confirmed in an experiment by Cornell physicists. Their work opens the door to new kinds of sensors and perhaps new ways to manipulate subatomic events.

In the Ultracold Lab of Mukund Vengalattore, assistant professor of physics, researchers observed a mass of atoms cooled almost to absolute zero so that they could "tunnel" from place to place. As long as the observation continued, no tunneling occurred.

This so-called "Zeno effect," named for the Greek philosopher who said that an arrow could never reach its target because it would always have half the distance yet to travel, derives from the proposal in 1935 by Austrian physicist Erwin Schrödinger that a subatomic event such as the decay of an atom remains undetermined until it is measured. He illustrated with a thought experiment involving the much-joked-about Schroedinger’s Cat. It follows that once you have measured something, it is locked in until you measure it again; so as long as you keep watching it can’t change.

Previous experiments have demonstrated the Zeno effect with the "spins" of subatomic particles, but "This is the first macroscopic demonstration, with real space measurements of the positions of atoms," Vengalattore said. The experiment is described in the Oct. 2 issue of the journal Physical Review Letters.

The researchers cooled a mass of about a million rubidium atoms into a "Bose-Einstein Condensate" (BEC), a state where the atoms are linked together and behave like a single particle, and suspended the mass in a laser beam inside a cold chamber. Inside a BEC the atoms link into an orderly lattice just as they would in a crystalline solid. And they can move around in that lattice: The famous Heisenberg uncertainty principle says that the position and velocity of a particle interact. Temperature is a measure of a particle’s motion. Under extreme cold velocity is almost zero, so there is a lot of flexibility in position; atoms can "tunnel" from one part of the lattice to another.

Too observe the lattice, the researchers placed a powerful microscope inside the cold chamber and illuminated the mass of atoms with another laser. A light microscope can’t see individual atoms, but a laser causes them to fluoresce, and the microscope can see the flashes of light from the atoms.

As predicted, under continuous observation the atoms don’t change position. And as the intensity of the observation beam increased, the amount of tunneling decreased proportionally.

"This gives us a way to control the system," said graduate student Yogesh Patil, lead author of the paper. The system can be very sensitive to nearby influences, he added, and could be used to create very delicate sensors - including one that would measure minute changes in gravity. Subtle changes in gravity from place to place across the Earth’s surface could identify subsurface mineral deposits, he noted.

The popular press has drawn a parallel with the "weeping angels" depicted in the "Dr. Who" television series - alien creatures who look like statues and can’t move as long as you’re looking at them. There may be some sense to that. "Quantum events don’t happen with humans because we’re under constant observation," Patil said. Or perhaps the folk wisdom is true: "A watched pot never boils."

Recent Ph.D. graduate Srivatsan Chakram, now at the University of Chicago, is the third co-author of the paper.

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